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Several battery technologies exist amongst other available electric energy storage technologies for both large and small-scale energy storage applications. Lead-acid and Li-ion batteries are presently the two most widely used battery storage technologies for small scale applications. Though environmental temperature greatly affects the operation performance of these two battery technologies, each has temperature range which it is more adaptable to. This paper reviews literatures on battery energy storage aspect of electrical energy storage technologies and literatures on world climatic characteristics to deduce that the comparative battery energy storage technology for tropical region is Lithium-ion battery.
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2017 IEEE 3rd International Conference on Electro-Technology for National Development (NIGERCON)
Review Of Comparative Battery Energy Storage
Systems (Bess) For Energy Storage Applications In
Tropical Enviroments
Agwu Daberechi D., Opara F. K., Chukwuchekwa N, Dike. D. O., Uzoechi L.,..
Department of Electrical and Electronic Engineering,
Federal University of Technology, Owerri. (FUTO), Nigeria.
Abstract Several battery technologies exist amongst other
available electric energy storage technologies for both large and
small-scale energy storage applications. Lead-acid and Li-ion
batteries are presently the two most widely used battery storage
technologies for small scale applications. Though environmental
temperature greatly affects the operation performance of these
two battery technologies, each has temperature range which it is
more adaptable to. This paper reviews literatures on battery
energy storage aspect of electrical energy storage technologies
and literatures on world climatic characteristics to deduce that
the comparative battery energy storage technology for tropical
region is Lithium ion battery.
Keywords battery, lead-acid, lithium-ion, energy storage,
temperature, tropic
One of the major challenges of electricity generation and
consumption using renewable energy resources especially,
solar and wind is their periodic generation discontinuity.
Whilst solar availability can be easily predicted, wind energy
is quite stochastic. This periodic availability makes it difficult
to solely rely on solar or wind electric power generation for
constant power needs. According to [1], this same periodic
availability challenge as well as the fact that its energy
generation is not easily controlled by the system operator,
make it more difficult to integrate solar and wind plants in the
generation pool compared to conventional (hydro, gas,
nuclear, etc) power plants. To overcome this challenge, the
use of electrical energy storage devices is being presently
extensively applied. Electrical Energy Storage (EES)
technology as described by [2] and [3] is the process of
converting electrical energy to a storable form and reserving it
in various media; then the stored energy can be converted
back into electrical energy when needed. Based on electrical,
mechanical, chemical and thermal energy storage principles,
[4] and [5] highlighted that a number of storage technologies
are available with quite different technical parameters and
operating characteristics.
Following the importance of electrical energy storage
systems towards achieving reliability in both renewable and
convectional power systems, scholars keep researching and
developing EES for use in both large and small-scale
electricity generation. Battery Energy Storage (BES) system is
an aspect of EES. Ease of use and scalability to achieve
desired energy storage capacity makes BES a global top
choice for small scale solar and wind power applications.
There are many commercially available battery technologies,
but each has advantages and limitations based on specific
application, environmental factors, power and energy scale,
cost, availability, safety issues, etc. For the purpose of
comparing suitable BES technologies for tropical regions, the
sections below critically review scholarly works on EES
technologies; with emphasis on BES systems.
According to [6], [2] and [3], the several methods
suggested for categorization of various EES technologies
include their functions, response times, and suitable storage
durations. Researchers in [7] and [8] classified according to
form of energy stored as shown in Fig. 1 while [9] and [1]
classified according to form of application as shown in Fig. 2.
The classification by [9] reveals that BESS can be applied
in both small and large-scale applications. A battery is a
combination of electrochemical cells connected in series or
parallel which produces electricity at a designed voltage using
electrochemical reactions. As explained by [10], [11] and [3],
a cell basically consists of two electrodes (one anode and one
cathode) with either solid, liquid or gel states electrolyte.
Depending on phase, a cell bi-directionally converts energy
between electrical and chemical energy. In discharging phase,
electrochemical reactions take place simultaneously at a cell’s
anode and cathode to release and drive electrons through the
external circuit from anodes and to cathodes. In the charging
phase, appropriate external voltage is applied across the two
electrodes to cause reverse reactions and make the battery
Batteries have relatively low cycling times and high
maintenance costs. These factor as pointed out by [3] makes
batteries to be rarely used for implementing large scale
facilities despite the fact that it has over time proved to be an
excellent electrical energy storage (EES) device.
2017 IEEE 3rd International Conference on Electro-Technology for National Development (NIGERCON)
Fig. 1. Classification of EES technologies by the form of stored energy [3]
Figure 2: Classification of EES technologies by the form of application [1]
Another factor as highlighted by [7] is dependency of
battery lifespan on its Depth-of-Discharge (DoD) cycle. This
makes complete discharge of battery inadvisable. The
following subsections describes some important battery
A. Leadacid batteries
In practical implementation of battery technologies, [12]
and [2] presented the LeadAcid (LA) battery as the most
widely used rechargeable battery. LA batteries are widely used
in stationary devices as back-up power supplies for computing
devices, telecommunication systems and energy management
in utility facilities; as well as hybrid or full electric vehicles.
The cathode of LeadAcid (LA) battery is made of PbO2
while its anode is made of Pb, and the electrolyte is sulphuric
(H2SO4) acid. Authors in [13], [14] and [2] all described
attribute of LA battery to include fast response time, small
daily self-discharge rates (<0.3%), relatively high cycle
efficiencies (63-90%) and low capital costs. Other attributes
include energy density (5090Wh/L) and specific energy (25
50 Wh/kg). Just as [3] and [8] mentioned, [15] and [16]
respectively pointed out that the application of Lead Acid
battery for large utility applications is still limited because LA
battery have relatively low cycling time (up to about 2000).
Besides, Leadacid battery performance is temperature
dependent. Thus for optimal performance, a thermal
management system is normally required which of course
increases cost of the entire system.
Currently, researches for development of better LA
batteries seems to be focusing on:
(a) Materials research for performance improvement in
areas such as extending cycling times and enhancing the deep
discharge capability.
(b) Implementation strategies for enhanced performance in
the wind, photovoltaic power integration and automotive
sectors applications.
B. Lithium-ion (Li-ion) batteries
The cathode of Li-ion battery as explained by [17] and [3]
is made of a lithium metal oxide such as LiCoO2 and LiMO2
while the anode is made of graphitic carbon. The electrolyte is
non-aqueous organic liquid containing dissolved lithium salts,
such as LiClO4. According to [6], [14] and [2], Li-ion battery
have their response time, energy and power density in the
order of milliseconds, 75200Wh/kg and 1502000W/kg
respectively. They further stated in their publications that Li-
ion batteries have high cycle efficiencies, up to 97%.
Reference [3] highlighted that the main drawbacks of Li-ion
BES system are that Depth of Discharge (DoD) cycle could
affect the Li-ion battery’s lifetime and that the battery pack
usually requires electronic controllers to manage its operation.
This of course increases its overall cost. The performance also
depends on operating temperature just like LA battery. Current
researches on Li-ion battery as pointed out by [3] focuses on:
(1) Increasing battery power capability with the use of
nano-scale materials
2017 IEEE 3rd International Conference on Electro-Technology for National Development (NIGERCON)
(2) Enhancing battery specific energy by developing
advanced electrode materials and electrolyte solutions.
(3) Implementation strategies/management systems for
enhanced performance
C. Sodiumsulphur (NaS) batteries
In their publication, [18] explained that a Sodiumsulphur
(NaS) battery uses molten sodium and molten sulphur as its
two electrodes and uses solid electrolyte referred to as “beta
alumina”. They further explained that the reactions normally
require a temperature of 574624 K to ensure that the NaS
battery electrodes remain in molten state in order to maintain a
high reactivity.
The advantages of Sodiumsulphur (NaS) battery as
highlighted by [6] and [17] include relatively high energy
densities (150300Wh/L), almost zero daily self-discharge,
higher rated capacity than other types of batteries (up to
244.8MWh) and high pulse power capability. It was further
revealed in [17] and [2] that NaS battery uses inexpensive,
non-toxic materials leading to 99% recyclability. Sodium
sulphur (NaS) is not commercially available in very small
scale because it has very high annual operating cost. Besides,
extra heating system is required to guaranty its operating
temperature. According to [3], NaS battery is considered as
one of the most promising candidates for large scale, high
power EES applications.
D. Nickelcadmium (NiCd) batteries
Authors in [3] explained that a NiCd battery uses nickel
hydroxide and metallic cadmium as the two electrodes and an
aqueous alkali solution as the electrolyte. NiCd battery has
relatively high robust reliabilities and low maintenance
requirements. However, the weaknesses of NiCd batteries as
pointed out by [19] include: environmental hazards and
pollution because cadmium and nickel are toxic heavy metals,
the battery capacity can be dramatically decreased if the
battery is repeatedly recharged after being partially
discharged. Till date there have been very few commercial
successes using NiCd batteries for large scale EES
applications. Thus [3] is of the opinion that NiCd batteries will
unlikely be heavily used for future large-scale EES projects.
E. Other battery energy storage technologies
In [20], the Nickelmetal Hydride (NiMH) battery was
extensively discussed. NiMH is similar to NiCd battery except
that a hydrogen-absorbing alloy is used as the electrode
instead of cadmium. NiMH battery has moderate specific
energy (70100Wh/kg) and relatively high energy density
(170 - 420Wh/L) that is significantly better than those of the
NiCd battery. Compared to NiCd, NiMH batteries are more
environmental friendly and has longer life cycle compared
with Li-ion batteries. However, [3] and [20] did point out that
NiMH has very high rate of self-discharge, losing 520% of
its capacity within the first 24hrs after full charging. They also
highlighted that NiMH is sensitive to deep cycling the
performance decreases after a few hundred full cycles.
Sodium Nickel Chloride (NaNiCl2) battery commonly
referred to as ZEBRA battery is described to have similar
properties to that of NaS battery by [21], [22] and [2]. The
specific energy of NaNiCl2 battery which is about 94
120Wh/kg is moderate. NaNiCl2 has energy density of
150Wh/L, specific power (150170 W/kg), and a high
operating temperature that ranges from 523K to
623K.Researchers in [3] presented NaNiCl2 battery as having
the advantages of good pulse power capability, cell
maintenance free, very little self-discharge and relatively high
cycle life.
The drawback of NaNiCl2 battery as indicated in [3] is that if
the electrodes solidifies, it takes 1215 h to heat up the
electrodes of the battery back to molten. Only few companies
have been involved in the development of this technology and
have produced this type of battery, thus limiting its
commercial availability. Table 1 is an excerpt of battery
energy storage systems summarized alongside other Electrical
Energy Storage (EES) systems by [3].
Life time
5 15
5 - 15
10 - 15
10 -
1000 -
2000 -
70 80
90 - 97
75 - 90
60 -70
0.1 - 0.3
0.1 - 0.3
~ 0
0.2 -
Tens of
50 80
200 -
150 -
10 400
1500 -
140 -
80 -
The performance of Lead acid battery and Li-ion battery
are affected by environmental temperature. Upon critical
review of the characteristics of the various battery energy
technologies, one may wonder why Lead acid batteries are
the most widely used despite their relatively low cycle time,
2017 IEEE 3rd International Conference on Electro-Technology for National Development (NIGERCON)
cycle efficiency, energy and power densities. This puzzle was
answered indirectly by [3] in the maturity status indicated as
status of the technology. Presently, Lead acid battery
technology is said to be technically matured, meaning it has
been developed to the extent that it is very acceptable.
The optimal operating temperature range for lead acid
battery is 20 to 25° C. In their tips for maximizing battery life
in solar application, [23] stated that for every 10° C (1F)
rise above room temperature (25 °C or 77 °F), battery life
decreases by 50%. Hence, [23] advises users to store and
operate batteries in a cool, dry place. Authors in [24] in their
technical report revealed that the life of a battery at 25°C is
71%, at 30°C it is 50% and at 40°C the life is only 25%. The
implication is that if a battery which originally has a 10years
lifespan is kept at 40°C, the effective ‘design life’ would be
2.5 years and performance would drop below 100% before 2
According to [25], Lithium-ion performs well at elevated
temperatures but prolonged exposure to heat reduces
longevity. Manufacturers of Li-ion battery usually gives the
operating temperature of lithium ion battery to range from 0
to 45°C for charging operations and 20 to 60°C for
discharging operations. However, in their report [26] claims
that the optimal temperature range for lithium ion battery
operation is between 15 to 35°C. Fig. 3 is a graphical
summary of [26] analysis.
Fig 3: Optimal operating temperature of Li ion battery [26]
The rate of chemical reactions to produce electric power in
Li-ion batteries reduces as temperature decreases. According
to [27], [28] and [29], decrease in temperature causes
contraction of the electrode materials; which slows down ion
movements in the intercalation spaces. As this continues, the
electrodes will stop accommodating current flow thereby
resulting in reduced power output as well as irreversible
capacity loss due to Lithium plating of the anode. On the other
hand, operating Li-ion battery at high temperatures gives rise
to a different set of problems which can result in the
destruction of the cell. First, the magnitude of available power
and capacity start decreasing after 35°C according to [26].
However, [30] has it that operating Li-ion battery slightly
above 50 ºC also reduces cycle life and at above 70 ºC the
threat of thermal runaway arises. Fig. 4 is a graphical
summary of temperature Storage capacity relationship
analysis as presented by [30].
Tropical regions are simply geographical areas within the
farthest points around the equator where the sun can be
directly overhead. Tropical regions are bounded by the two
23˚ 27’ parallel latitudes in north and south of the equator
commonly referred to as tropic of Cancer and Capricorn,
respectively. A tropical climate has warm temperatures
throughout the year
Fig. 4: Temperature Storage capacity relationship [30]
and a significant amount of precipitation. Nigeria has tropical
climate. According to a report by [31], the maximum
temperatures during the hot season (February and March) in
the south and (March and April) in the north ranges between
30.1 40.0°C. Reference [31] further stated that the southeast
coastal areas had mean maximum temperatures of 30.0
33.0°C, while the coast of the southwest and the inland cities
of the south recorded maximum temperatures between 33.0
36.0°C. Maximum temperatures over the central areas and the
north central parts ranged between 36.0° -38.0°C during hot
period except for Jos and its environs which had mean
maximum temperature ranging between 30° -36°C. Jos
recorded the lowest maximum temperature of 30.1°C.
Elsewhere in the north, maximum temperatures were between
38.0° - 40.0°C.
The minimum temperature recorded at extreme northern
parts of Nigeria happened to be below 18˚C which is a general
benchmark for tropical regions. Table 2 is a summary of
maximum northern daily temperatures for the year 2012.
2017 IEEE 3rd International Conference on Electro-Technology for National Development (NIGERCON)
(oC )
February - May
March - May
February - May
February - May
March - May
February - May
March - May
February - May
April - May
March - May
Tthe battery energy storage systems (BESS) have their
merits and demerits, the literatures reviewed in this work
reveal that Lithium ion battery is at the moment the best
energy storage technology for tropical environments. This is
due to the facts that their optimal operating temperature range
is just about the same with the average daily environmental
temperature of the tropics.
For instance, the minimum optimal operating temperature
of lithium-ion battery as given by [26] is 15.0°C while the
average minimum temperature of tropical region is 18.0°C.
Thus Lithium-ion batteries will not have cause to face its
extreme low temperature challenges as pointed out earlier
whilst operating in tropical areas. Likewise, the maximum
optimal temperature by [26] is 35.0°C which is above the
maximum temperature of many tropical region especially the
rainforest. Moreover, the temperature 60.0°C at which [30]
and [26] indicated that Li-ion battery will experience thermal
runaway and drastically drop its capacity is well above the
maximum recorded temperature of 43.7°C in Nigeria as a
tropical region.
Though Li-ion battery requires thermal management just
like lead-acid battery and other battery energy storage
systems, making Li-ion battery a choice for tropical regions
like Nigeria will reduce the efforts and resources channeled
towards battery thermal management. For instance, there will
be no need to heat up its immediate environment during cold
times and also less need to cool the immediate environment
during hot times as compared with lead-acid battery.
The two most widely used battery technology (Lead-acid
and Li-ion) for small scale energy storage applications have
temperature challenge in common. Whilst lead-acid battery
adapts better to lower temperatures suiting temperate climate,
Li-ion battery adapts more to higher environmental
temperature relative to that of tropical climates. If the concept
of comparative advantage will be borrowed in this subject; it
is hereby deduced in accordance with the reviewed literatures
that the comparative battery energy storage technology for
tropical region is Lithium ion battery.
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... The rate at which lithium ion batteries degrade is not only closely associated with the cell chemistry and design, but also to the operating conditions to which it is subjected. In general, most lithium ion battery chemistries have an ideal working temperature range of 15e35 C [3]. The battery management system (BMS) regulates the temperature of each cell to stay in this range in order to prevent the battery from overheating or freezing. ...
Full-text available
An ageing test procedure was developed in this work in order to assess the influence of battery degradation effects, occurring during electrical cycling of lithium-ion batteries at elevated temperatures, on the electrical and mechanical properties of the cells and on their safety behaviour under mechanical quasi-static crush loading. Commercial 41 A h NMC-LMO/graphite pouch cells were charged and discharged at 60 °C for 700 cycles at 1 C in an SOC range between 10 and 90%. Electrical properties of these batteries were evaluated every 100 cycles at room temperature. By the end of the cycling procedure, a 27% reduction of the initial capacity was observed. The behaviour of the fully charged aged cells under a quasi-static mechanical load was examined in a series of indentation tests, using a flat impactor geometry. This behaviour was compared to that of fresh cells. Test results show that all investigated electrically cycled cells exhibited a slight decrease in stiffness. Aged batteries also failed at higher compressive strengths and larger deformations. Post-mortem analysis of the aged cells was performed as a next step using scanning electron microscopy and the occurred degradation effects were evaluated in order to explain the changes observed in the battery electrical and mechanical properties.
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High-temperature superconductor (HTS) based devices have the potential to be useful technologies for space applications, allowing very high current transfer and magnetic field generation. However, HTS technology requires cryogenic temperatures (<90 K) to operate, and it is not well understood if and how this can be achieved for HTS devices integrated into space vehicles. In this study, the thermal and power performance of a hypothetical 3U CubeSat equipped with an HTS magnetic coil is explored over a range of orbits around the Earth. After eliminating the possibility of passive cooling this close to a planetary body, a cryocooler was deemed necessary to maintain the required temperatures and was included in the simulations. The results show that the best strategy for maintaining the cryogenic operating environment is to maximise the power availability to the cryocooler from solar panels. This approach increases the volume-averaged temperature of the satellite, but the benefits of increased power outweigh the cost of a decreased cryocooler efficiency. This work demonstrates that a 1 T magnetic field can be generated with an HTS electromagnet in a space environment on a small satellite, enabling the use of HTS for space applications such as electric propulsion and energy storage.
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The paper gives an overview of the innovative field of hybrid energy storage systems (HESS). An HESS is characterized by a beneficial coupling of two or more energy storage technologies with supplementary operating characteristics (such as energy and power density, self-discharge rate, efficiency, life-time, etc.). The paper briefly discusses typical HESS-applications, energy storage coupling architectures, basic energy management concepts and a principle approach for the power flow decomposition based on peak shaving and double low-pass filtering. Four HESS-configurations, suitable for the application in decentralized PV-systems: a) power-to-heat/battery, b) power-to-heat/battery/hydrogen, c) supercap/battery and d) battery/battery, are briefly discussed. The paper ends with a short description of the HESS-experimental test-bed at Chemnitz University of Technology.
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Electrical power generation is changing dramatically across the world because of the need to reduce greenhouse gas emissions and to introduce mixed energy sources. The power network faces great challenges in transmission and distribution to meet demand with unpredictable daily and seasonal variations. Electrical Energy Storage (EES) is recognized as underpinning technologies to have great potential in meeting these challenges, whereby energy is stored in a certain state, according to the technology used, and is converted to electrical energy when needed. However, the wide variety of options and complex characteristic matrices make it difficult to appraise a specific EES technology for a particular application. This paper intends to mitigate this problem by providing a comprehensive and clear picture of the state-of-the-art technologies available, and where they would be suited for integration into a power generation and distribution system. The paper starts with an overview of the operation principles, technical and economic performance features and the current research and development of important EES technologies, sorted into six main categories based on the types of energy stored. Following this, a comprehensive comparison and an application potential analysis of the reviewed technologies are presented.
Electricity from renewable sources of energy is plagued by fluctuations (due to variations in wind strength or the intensity of insolation) resulting in a lack of stability if the energy supplied from such sources is used in 'real time'. An important solution to this problem is to store the energy electrochemically (in a secondary battery or in hydrogen and its derivatives) and to make use of it in a controlled fashion at some time after it has been initially gathered and stored. Electrochemical battery storage systems are the major technologies for decentralized storage systems and hydrogen is the only solution for long-term storage systems to provide energy during extended periods of low wind speeds or solar insolation. Future electricity grid design has to include storage systems as a major component for grid stability and for security of supply. The technology of systems designed to achieve this regulation of the supply of renewable energy, and a survey of the markets that they will serve, is the subject of this book. It includes economic aspects to guide the development of technology in the right direction. • Provides state-of-the-art information on all of the storage systems together with an assessment of competing technologies • Features detailed technical, economic and environmental impact information of different storage systems • Contains information about the challenges that must be faced for batteries and hydrogen-storage to be used in conjunction with a fluctuating (renewable energy) power supply.
With the rapid growth of wind energy development and increasing wind power penetration level, it will be a big challenge to operate the power system with high wind power penetration securely and reliably due to the inherent variability and uncertainty of wind power. With the flexible charging-discharging characteristics, Energy Storage System (ESS) is considered as an effective tool to enhance the flexibility and controllability not only of a specific wind farm, but also of the entire grid. This paper reviews the state of the art of the ESS technologies for wind power integration support from different aspects. Firstly, the modern ESS technologies and their potential applications for wind power integration support are introduced. Secondly, the planning problem in relation to the ESS application for wind power integration is reviewed, including the selection of the ESS type, and the optimal sizing and siting of the ESS. Finally, the proposed operation and control strategies of the ESS for different application purposes in relation to the wind power integration support are summarized. The conclusion is drawn in the end.
This review describes the advantages and characteristics of employing polymer electrolytes in solid-state lithium-ion batteries. Criteria for an ideal polymer electrolyte and the differences between polyelectrolytes are discussed. The emphasis of this article is on plasticized or gelled electrolyte systems. Hence, the review focuses on four plasticized systems which have received particular attention from a practical viewpoint, i.e., poly(ethylene oxide) (PEO)-, poly(acrylonitrile) (PAN)-, poly(methyl methacrylate) (PMMA)-, and poly(vinylidene fluoride) (PVdF)-based electrolytes. Some critical concepts and points associated with this emerging technology that still require attention are discussed in the final part of the review.
The Ni–MH batteries were tested for battery energy storage characteristics, including the effects of battery charge or discharge at different rates. The battery energy efficiency and capacity retention were evaluated through measuring the charge/discharge capacities and energies during full and partial state-of-charge (SoC) operations. Energy efficiency results were obtained at various charge input levels and different charge and discharge rates. The inefficient charging process started to take place at ca. 90% state-of-recharge (SoR) when charged at no more than 0.2 C rate. For the NiMH-B2 battery after an approximately full charge (∼100% SoC at 120% SoR and a 0.2 C charge/discharge rate), the capacity retention was obtained as 83% after 360 h of storage, and 70% after 1519 h of storage. The energy efficiency was decreased from 74.0% to 50% after 1519 h of storage time. The Coulomb efficiency was initially 83.34%, and was reduced to 57.95% after 1519 h of storage. The battery has relatively higher energy efficiency at approximately 50% SoC. The energy efficiency was calculated to be more than 92% when the NiMH-C3 battery was charged to 30–70% SoC then discharged to 0% SoC at a 0.2 C charge/discharge rate. In consideration of energy efficiency, charge acceptance, capacity retention rate, and power output needs, as well as Nelson’s analysis on HEV power requirements, the Ni–MH battery is appropriate to work at ca. 50 ± 10% SoC with an operating limitation of 50 ± 20% SoC. This work is potentially beneficial for determination of the current SoC level during the battery pack being operated for energy storage applications.
The effect of solute–solvent interactions on the viscosity of dilute electrolyte solutions is considered in terms of the solut contribution to the activation free energy, enthalpy or entropy, for viscous flow. Two classes of electrolyte are considered those involving inorganic ions and those involving larger organic ions such as the tetraalkylammonium salts. The solute contributions to the activation free energies for viscous flow Δμ2θ‡ for inorganic electrolytes vary systematically with the charge and radius of the ion in a way that is consistent with a primaril electrostatic effect, increasing linearly with the ionic charge and with the inverse of the ionic radius. In contrast, th Δμ2θ‡ values for the tetraalkylammonium halides increase linearly with the cationic volume. For both types of electrolytes, Δμ2θ‡ in organic solvents is relatively insensitive to the solvent, while the values for aqueous systems are markedly differen from those in the organic solvents.
ZEBRA batteries use plain salt and nickel as the raw material for their electrodes in combination with a ceramic electrolyte and a molten salt. This combination provides a battery system related specific energy of 120Wh/kg and a specific power of 180W/kg. With these data the battery is well designed for all types of electric vehicles and hybrid electric buses. The ZEBRA battery technology is industrialised in Switzerland where a new plant has a capacity of 2000 packs a year with expansion prepared for 30,000 packs a year.
A symmetric cell was adopted to analyze low temperature performance of Li-ion battery. Results showed that impedances of both Li-ion and symmetric cells are mainly composed of bulk resistance (R b), surface layer resistance (R sl) and charge-transfer resistance (R ct). Among these three components, the R ct is most significantly increased and becomes predominant as the temperature falls to below À10 8C. Therefore, we may ascribe the poor low temperature performance of Li-ion battery to the substantially high R ct of the graphite and cathode. Comparing impedance spectra of the symmetric cells, we found that at À30 8C the delithiated graphite and lithiated cathode, both of which correspond to a discharged state in a Li-ion battery, have a much higher R ct than when charged. This means that the Li-ion battery in the discharged state suffers a higher polarization. This result explains the phenomenon that at low temperatures, charging of a discharged Li-ion battery is more difficult than discharging of a charged battery. # 2002 Elsevier Science B.V. All rights reserved.