Three‐stage kinetics of laser‐induced LiFePO₄ decomposition

Abstract

This paper reports about new insight into a problem of a laser–matter interaction during Raman probing of lithium iron phosphate (LiFePO4), discusses phase transformation kinetics in powder samples, and provides some methodological recommendations. LiFePO4 is the second most popular positive electrode material in the global lithium battery industry, but the use of Raman spectroscopy for its structural characterization is hampered by the laser‐induced degradation. The statistical/big‐data approach to Raman spectra measurements is utilized to revise the problem and suggest a simple model of laser‐induced phase transformations in powder LiFePO4. The results are proposed to be used for better understanding of the physics and chemistry of other processes taking place in electrochemical cells, namely, lithiation/delithiation and thermal stability/safety studies. Simple recommendations for nondestructive LiFePO4 characterization by Raman spectroscopy are formulated.

An ambiguity of laser‐induced degradation in LiFePO4 and advantages of single‐particle approach to Raman spectroscopy

“…One more point worth mentioning is that each act of laser‐induced decomposition observed by Ryabin et al [ 16 ] was followed by the propagation of decomposition front to the nearby areas due to heat release during sample oxidation. But in Figure 2d, one can see that the decomposition of the smaller particle did not affect the adjacent larger one, which retained the LFP structure.…”

“…These bands are not mentioned in any publication on laser‐induced LFP degradation. [ 14–16,26,28,29,33 ] Impurities as a possible reason for different decomposition products are unlikely because, in this work, we studied just the same LFP sample as in the work of Ryabin et al [ 16 ] None of several hundred Raman spectra of decomposed LFP, measured by Ryabin et al, [ 16 ] were characterized by a triplet in the spectral range 900–1100 cm −1 . None of several hundred LFP spectra without decomposition contained notable impurity phases, so we assume that these unidentified spectra in this work are attributed to unknown LFP decomposition products.…”

“…So far as oxidation needs oxygen, it initiates at the particle's surface. If the Raman spectrum is measured in powder, the released heat may trigger further oxidation as in the particle bulk, so in the adjusting particles, thus resulting in the decomposition front propagation, described by Ryabin et al [ 16 ] As a result, the decomposed particles heat each other. In the case of a nonequilibrium phase of Li 3 Fe 2 (PO 4 ) 3 , this overheating results in a phase transition to an equilibrium state.…”

“…Raman spectroscopy is widely used for the structural characterization of such materials. The laser‐induced phase transitions were reported for most main types of cathode materials: LiFePO 4 (LFP), [ 12–16 ] LiCoO 2 , [ 17 ] LiNi 1/3 Mn 1/3 Co 1/3 O 2 , [ 18 ] and 0.6Li[Li 0.33 Mn 0.67 ]O 2 –0.4Li[Mn 0.3 Ni 0.45 Co 0.25 ]O 2 . [ 19 ]…”

“…For the LFP/C sample, the authors reported the oxidative laser‐induced decomposition of LFP into γ‐Li 3 Fe 2 (PO 4 ) 3 and α‐Fe 2 O 3 . In 2020, Ryabin et al [ 16 ] performed a statistical Raman study of pure LFP and supposed three‐stage kinetics of laser‐induced LFP decomposition. The authors revealed a propagation of the decomposition front over the powder sample.…”