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An accelerated rate calorimeter in combination with a battery cycler and a precision multimeter was used to measure the heat dissipation from, and heat accumulated in, commercially available lithium-ion cells during cycling over a range of operating parameters within the limits recommended by the manufacturer. An integral energy balance was used to determine the total heat generated in the test cell during cycling. From the measurements during temperature transients 1 the heat capacity of the test cell was found to be relatively independent of temperature, ranging from 0.82 to 1.07 J g-K -'. This value agrees relatively well with separate measurements using an adiabatic calorimeter which yield slightly higher values. DC current interruption technique was used to determine the time-dependent area-specific impedance, , of the cell which was well correlated with steeply increased heat dissipation rate at the end of discharge. The reversible (entropic) heat effect derived from an energy balance was found to be exothermic during discharge and endothermic during charge. Using two different methods, values were obtained for the entropy of reaction (AS) during discharge of the cell. The resulting values obtained with method II, depending on the discharge rate, varied from -41.19 to -80.98 J K -' per g mole of Li and showed a weak dependence on temperature, in the 35 to 55"C range. The rate dependence of the AS values needs further examination in a future study. By extrapolating to zero rate, the reversible entropy of the faradaic reaction for this cell was found to be -37 _ 3 J Kl per g mole of Li within the temperature range. The entropic heat effect and the heat effect associated with nonfaradaic reactions are appreciable and should be included in thermal modeling.

The effective chemical diffusion coefficient of Li (D Li ϩ) and related kinetic properties for lithium intercalation/deintercalation in Li x CoO 2 were determined by the galvanostatic intermittent titration technique (GITT). D Li ϩ values reached a peak at ca. x ϭ 0.5 in Li x CoO 2 , and this maximum correlates with phase transitions in the lithium host electrode which were indicated by the enhancement factor (W) and area-specific impedance () variations. The phase dependence of the D Li ϩ values at about x ϭ 0.5 was investigated, and a decrease of D Li ϩ value with increasing temperature was found. The D Li ϩ behavior during cycling confirms that the phase transition is reversible.

Due to its high voltage, up to 4.5 V, and its capability to reversibly insert lithium, Li x CoO 2 has attracted considerable interest as a positive electrode (P.E.) material for rechargeable lithium-ion batteries, despite the high cost of cobalt. It has already found commercial application in secondary lithium-ion batteries. 1 The reduction/oxidation chemistry of a Li-ion cell can be illustrated as in Fig. 1 for a typical Li x CoO 2 /carbon rechargeable cell. During charging, the oxidized Li ϩ ions from the P.E. (Li x CoO 2 ) diffuse as well as migrate through the ion-conducting nonaqueous electrolyte, and then are reduced by insertion into the negative electrode (N.E., carbon layers). This process is reversed during discharge. 2,3 Therefore, the lithium content, 1-x and x, of the active electrode materials Li 1Ϫx CoO 2 and Li x C, respectively, changes during charging and discharging as shown in Fig. 1. It may be expected that the thermal and/or electrochemical properties of the electrode materials also change with x.The relationship between the electrochemical and structural properties of Li x CoO 2 as a function of x has been studied in detail by Reimers and Dahn 4,5 using electrochemical extraction of Li and in situ X-ray diffraction (XRD) techniques. Their measurements indicated a sequence of three distinct phase changes as x was electro-chemically varied over the range 0.4 Յ x Յ 1. They found clear evidence for a first-order transition (coexisting H 1 and H 2 phases) in the range of 0.75 Յ x Յ 0.93, as shown in a phase diagram (Fig. 2 modified from Reimers and Dahn's work 4 ). This transition involved a significant expansion of the c lattice parameter of the hexagonal unit cell. In addition, lithium ordering 6-8 was evident near x ϭ 0.5, and this was found to be coupled to a lattice transformation in which there was a transition from hexagonal (H) to monoclinic (M) symmetry of the host lattice (Fig. 3). This phase transition was found to be temperature-dependent when lithium was deintercalated electrochemically from Li x CoO 2 over the temperature range Ϫ10ЊC Յ x Յ 60ЊC. 4 Such order/disorder transitions have also been reported for alkali-metal insertion compounds such as Li x TiS 2 , 9,10 2H-Li x TaS 2 , [11][12][13] and Li x Mn 2 O 4 . 14,15 As reported in a previous publication, 2 thermal behavior during discharge/charge cycling of Li x C 6 /Li 1Ϫx CoO 2 cells is complex and features minor opposite heat effects which appear to occur rapidly, one after the other, during the traversal of the transition range near x ϭ 0.5. In the earlier publication, an exothermic heat effect (heat generation) was tentatively ascribed to the transition from hexagonal to monoclinic phase and an endothermic heat effect (heat absorption) to the reverse transition. 2 A similar thermal behavior during a calorimetric study of a Li x C 6 /Li 1Ϫx CoO 2 cell has been reported by Saito et al., 16 and was ascribed to this type of phase transition.In this paper, we report the results of detailed electrochemicalcalorimetric measurements...

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