Cathode delithiation

Hence, the presence of trace impurities, which either pre-exist in pristine electrode and bulk electrolyte or are introduced during the handling of the sample, could profoundly affect the spectroscopic images obtained after or during certain electrochemical experiments. This complication due to the impurities is especially serious when ex situ analytic means were employed, with moisture as the main perpetrator. For cathode/electrolyte interfaces, an additional complication comes from the structural degradation of the active mass, especially when over-delithiation occurs, wherein the decomposition of electrolyte components is so closely entangled with the phase transition of the active mass that differentiation is impossible. In such cases, caution should always be exercised when interpreting the conclusions presented. 

In contrast to that of solvents, the effect of the electrolyte solute, LiPFe, on the thermal decomposition of the cathode, LiCo02, was found to be suppression instead of catalyzation. The SHR of a partially delithiated cathode was measured in a series of electrolytes with various salt concentrations, and a strong suppression of the self-heating behavior was found as the concentration of LiPEe increased above 0.50 M. The mechanistic rationale behind this salt effect is still not well understood, but the authors speculated that the salt decomposition coated the cathode with a protective layer that acted as a combustion retardant. On the basis of these results, the authors recommended a higher salt concentration (>1.50 M) for LiCo02-based lithium ion cells is preferred in terms of thermal safety. 

Scheme 27. Thermal Combustion of EC by a Fully Delithiated Spinel Cathode...

Figure 68. Nyquist plots of a charged lithium ion cell, a lithiated graphite/graphite cell, and a delithiated cathode/ cathode symmetrical cell. The inset is an equivalent circuit used for the interpretation of the impedance spectra. (Reproduced with permission from ref 512 (Figure 3). Copyright 2003 Elsevier.)...

Some metal oxide structures are unstable when over-delithiated, and as a consequence, the crystal lattice collapses to form a new phase that is electrochemically inactive. Examples are the so-called Jahn—Teller effect for spinel cathodes and similar behavior for LiNi02 and LiCo02 materials as well. These irreversible processes are considered to be caused by the intrinsic properties of the crystalline materials instead of electrolytes and are, therefore, beyond the scope of the current review. See ref 46 for a detailed review. 

Formation mechanism of SEI layers on cathodes in Li-ion batteries, their thermal and electrochemical stabihty, and their roles in affecting the cycle life and safety characteristics are well documented by many researchers [43 6]. Here we present some recent data on identifying the surface layer generation and their composition on transition metal oxide cathodes like spinel and layered materials by various spectroscopic techniques. The structural changes and the reaction at the surface during the first delithiation process in Li-rich layered material are explained. The effects of additives and coatings on electrode materials to their electrochemical performance are also discussed at the end. 

Stress within the cathode or anode materials can be caused when the cathode or anode is fully lithiated or delithiated . This effect, leading to battery durability issues, has to be understood and carefully assessed when deciding on the SOC operating strategy. 

FIGURE 20.14 DSC profiles of chemically delithiated Lio.49Co02 with electrolyte for various cathode weights in 3 nl electrolyte [22],... 

In the particular case of lithiation or delithiation of cathode materials used in lithium secondary batteries, the calculation of the electrochemical equivalent involves an additional parameter related to the reaction of intercalation of lithium cations into the crystal lattice of the host cathode materials. Consider the theoretical reversible reaction of intercalation of lithium into a crystal lattice of a solid host material (e.g., oxide, sulfide) ... 

LiCoOj is the most common cathode material used today in commercial Li-ion batteries by virtue of its high working voltage, structural stability and long cycle-life. However, Co is relatively expensive and the cheaper Mn material suffers from the instability problems described above. Much effort has therefore been made in recent years to find cheaper alternatives. LiNiOj (isostructural with LiCoO ) is a promising materials in this respect, but has not been commercialised successfully for several reasons i) difficult synthesis conditions, ii) poor structural stability on electrochemical cycling, and iii) poor thermal stability in its delithiated state as a result of the unstable Ni" ion. These problems can be circumvented by partially substitution of Ni by other cations, typically Co. The relative performances of the Li(Ni,Co)02 family of materials are compared with those of spinel in Table 4 ... 

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