Ragone Plots” of Lithium-Ion Batteries
2.1. Abstract
Lithium-ion batteries operating at high-low temperatures and high C rates pose a technical barrier for operating hybrid and electric vehicles due to substantial reduction in energy and power. This Chapter explains simultaneous effect of temperature and C rate on state-of-health, power and energy density of LIBs. Ragone charts are developed on LIBs based on LiCoO2 cathode and graphite anode chemistry at various temperatures between -20 and 55 oC, and C rates (C/5-10C). To account for varying power and temperature Ragone isotherms are integrated over the power range yielding a functional Integrated Energy-Power (2√∫ 𝐸(𝑃)𝑑𝑃𝑃 ∣𝑇) . The ratio of integrated areas at various temperatures to 25 C gives the “Energy-Power Index” (EPI) (%).
The EPI vs. T profile shows a linear increase between -10 C and 25 C then it makes a semi-plateau between 25 C and 55 C. Besides, for the optimum performance of LIBs operating at low temperatures, low C-rate application is most preferable.
72 2.2. Background and Motivation
The LIBs should have high energy density (E.D.) and power density (P.D.) to be used in hybrid and electric vehicles [1-7]. The E.D. is influenced by capacity and voltage of the battery. The E.D. and P.D. are inversely related. An ideal battery should have high E.D. and P.D. and should not be influenced by climatic conditions and the rate at which battery is cycled. The application of LIBs are limited because the electrochemical performance of the batteries are temperature dependent [8-19]. There are different climatic conditions in different parts of world where the temperature is lower or higher than ambient temperature (25 oC) [8-19]. There are many reports which states that operating LIBs at ambient temperature show excellent electrochemical performance compared to lower or higher temperatures [8-19].
Viscosity of carbonate based electrolytes (particularly ethylene carbonate (EC)), changes at low temperatures, leading to sluggish ion transport, promote high concentration polarization, yielding depleted electrolyte areas that could cause higher cell impedances and can lead to formation of EC-rich solid phases [12,15, 19, 20].
Besides, at low temperature polymeric components within the cell tend to become brittle [21]. The active electrodes materials tend to fall apart due to binder. All these factors lead to reduction in cell capacity and lifetime of the battery at low temperatures. At high temperatures, side reactions occur much faster, so battery degrades much earlier. Heating could cause thermal runaway of the cell and lead to flame or explosion [11, 17-18]. The life, energy and power density of LIBs decreases when operated at low or high ambient temperatures. Besides, under high C rate operations, the cell capacity decrease is due to fast local depletion of Li-ions in the electrolyte at the interface of electrode active materials and electrolyte by the EC reaction [19, 20, 22].
In overall, the major factors controlling the nominal energy of a battery are: i) state of health (SOH), ii) temperature, and iii) the drain power. SOH accounts for the ageing of the cell components including electrochemically active materials (anode, cathode, and electrolyte) [23] and inactive materials (separator, metal sheet substrates, electrode binder, additives, gasket, seals etc.) [24]. Ageing adversely affects the lithium storage capability of electrode materials owing to crystal structure degradation
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[25] and to inter-particles electrical disconnection [24]. Ageing also decreases the electrolyte conductivity leading to increased overall cell Ohmic resistance [26-29].
SOH is usually determined from the cell’ discharge profile at a well-defined temperature and discharge rate. The integration of the discharge curve (voltage vs.
time or vs. capacity) gives the energy provided by the cell. The energy of an aged cell is related to that of a fresh under the same discharge rate to yield the SOH (usually given in %).
Besides, temperature plays a major role in the cell energy density and power density. LIB discharge reaction is exothermic in nature (enthalpy variation H<0) [30]. Therefore, the amount of energy should decrease as temperature increases. On the other hand, the lithium ions diffusion in anode and cathode materials and their mobility in the electrolyte are thermally activated processes. This implies that within the range of the cell’ thermal stability the rate of charge and discharge should increase with temperature [31-32]. In general, the temperature dependence of energy has a bell shape starting with an increase up to a maximum and then decreases with temperature [33].
The charge and discharge rates also play a major role in the cell’s performance [34-35]. In fact, by increasing the rate, increases the anode and cathode polarization together with the Ohmic effect. Accordingly, with higher current rates the charge and discharge voltage limits are reached faster resulting in cell not being fully charged and fully discharged. This reduces the energy density and the charge-discharge cycle coulombic and energy efficiency. There is also a Joule effect which increases with the charge and discharge rate and at low temperatures leading to cell' heating [36].
Hence, there is an intense need to understand the change in electrochemical performance of the LIBs with change in temperature and rate so that we can find the minimum threshold temperature and rate for optimized performance of the battery.
Moreover, we can find the temperature where the performance is poor which helps in taking proper precautions for normal running of battery.
The energy vs. power "Ragone plots" [37-38] are convenient charts for comparing the energy and power densities of various energy storage devices and predicting the energy output under a well-defined power drain [39]. Ragone plots are
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usually achieved by discharging a fully charged cell (or battery pack) under a constant power and by integrating the voltage over the discharge capacity (Ah) to get energy.
The power (P) and energy (E) are related to the total mass or volume of the battery (W/kg or W/L and Wh/kg or Wh/L, respectively), which makes it possible to compare different battery systems as E vs. P or P vs. E charts.
In this chapter, we explain the change of energy and power densities of real lithium ion cells with change in temperature and C-rate by using Ragone plots [37- 38]. In this study, lithium ion cells based on LiCoO2 cathode and graphite anode initially fully charged at the ambient temperature (~25 °C) are discharged isothermally between -20 °C and 55 °C at different C-rates (C/5 to 10C). This enables Ragone plots to be achieved at different temperatures. To account for temperatures and rates variables we introduce a new metrics; the Power Energy Index (PEI). PEI (in %) is defined as the ratio of the root squared integrated area under the Ragone profile achieved at temperature T vs. the same at 25 ºC. The PEI vs. T profile shows a linear increase and semi-plateau. This feature is discussed in terms of trade-off between temperature effects on the thermodynamics and the kinetics of LIB.