Raman Microscopy of Lithium-Manganese-Rich Transition Metal Oxide Cathodes

Abstract

Lithium-rich and manganese-rich (LMR) layered transition metal (TM) oxide composites with general formula xLi 2 MnO 3 · (1-x)LiMO 2 (M = Ni, Co, Mn) are promising cathode candidates for high energy density lithium ion batteries. Lithium-manganese-rich TM oxides crystallize as a nanocomposite layered phase whose structure further evolves with electrochemical cycling. Raman spectroscopy is a powerful tool to monitor the crystal chemistry and correlate phase changes with electrochemical behavior. While several groups have reported Raman spectra of lithium rich TM oxides, the data show considerable variability in terms of both the vibrational features observed and their interpretation. In this study, Raman microscopy is used to investigate lithium-rich and manganese-rich TM cathodes as a function of voltage and electrochemical cycling at various temperatures. No growth of a spinel phase is observed within the cycling conditions. However, analysis of the Raman spectra does indicate the structure of LMR-NMC deviates significantly from an ideal layered phase. The results also highlight the importance of using low laser power and large sample sizes to obtain consistent data sets. Lithium-manganese-rich TM oxides form as the integrated structure of two layered phases: xLi 2 MnO 3 with C2/m space group symmetry and (1-x)LiMO 2 (M = Co, Mn, Ni, Fe, Cr) with R3m space group symmetry. While the exact structure of lithium-rich TM oxides has been the subject of much debate, a number of recent studies conclude that the material forms as the composite of these two distinct phases with nanoscale domains of Li 2 MnO 3 and LiMO 2 character. 1-6The intense interest in lithium excess cathodes stems from the potential to deliver very large reversible capacities (>200 mAh g −1 ) when charged beyond 4.5 V vs. Li 0 /Li + . 7 Part of the capacity derives from electrochemical activation of the Li 2 MnO 3 component at voltages beyond 4.4 V. Two mechanisms have been proposed for this activation. The dominant mechanism appears to be the simultaneous removal of lithium and oxygen to form a composition similar to MnO 2 . [8][9][10][11] Protons from electrolyte decomposition may also exchange for lithium. This secondary mechanism becomes more significant at higher temperature.12-14 Despite the promise of lithium rich TM oxides to deliver very high capacities, one significant limitation remains the large voltage and capacity fade observed upon extended electrochemical cycling. 15,16 A number of studies have attempted to link structural changes in lithium-rich TM oxides with electrochemical stability and cycling behavior.9,17-25 In particular, several groups have applied Raman microscopy to understand the structure and structural evolution of lithium-rich, manganese-rich TM oxides of nickel, manganese, and cobalt (hereafter LMR-NMC). 18,19,24,[26][27][28][29][30][31] Raman microscopy offers high spatial resolution (< 1μm 3 ), large field of view, and chemical specificity, making it an ideal tool to investigate both the pristine material and comp...

A Raman-Based Investigation of the Fate of Li2MnO3in Lithium- and Manganese-Rich Cathode Materials for Lithium Ion Batteries

“…The recent ex situ Raman study of Toda HE5050 cathodes in the pristine and cycled state by Ruther et al 9 and our subsequent study as reported herein are similar in many ways. Their spectra were recorded using 532 nm excitation; they took great care to record spectra under conditions that did not damage the LMR-NMC phase; and they avoided exposure of their extracted cathodes to ambient atmosphere before/during their Raman measurements.…”

“…Kumar-Nayak et al 8 interpreted their LMR-NMC Raman results in terms of a gradual layered to spinel transformation brought on by continuous electrochemical cycling. In a very recent paper Ruther et al 9 conducted Raman examinations of LMR-NMC cells cycled from a discharge voltage of 2.0 V to charging voltages as high as 4.9 V. There results indicated that the structure of the cycled cathode material deviates significantly from the ideal two-component layered phase present in the pristine LMR-NMC cathode. They also highlight the importance of laser power control (to avoid damaging the sample) and sampling over a wide area to assure the gathering of consistent data.…”

“…A number of pertinent studies, particularly some of the more recent ones, have employed Raman spectroscopy to elucidate the chemical and structural transformations that the LMR-NMC composition undergoes during electrochemical cycling in cells with lithium-based anodes. [6][7][8][9][10][11][12][13] Amalraj et al 6 used a combination of XRD, TEM, and ex situ Raman spectroscopy to establish that a partial structural transition from a layered-type to a spinel-type of ordering occurred in the initial charge to 4.7 V when their xLi 2 MnO 3 · (1-x)LiMO 2 cathode (M = Ni-Mn-Co) was cycled * Electrochemical Society Active Member.…”

Toward Low-Cost, High-Energy Density, and High-Power Density Lithium-Ion Batteries

“…14 Other techniques like Raman spectroscopy have also been used to identify variations in coating composition during off-line material testing. 15,16 These techniques are sensitive enough for certain defects like nonuniform coating and pinholes but may not have enough resolution to detect flaws like agglomerates and metal contamination in the electrode manufacturing process that lead to faulty cells being built, thereby increasing production cost.…”

Raman Spectroscopy for LiNi_1/3Mn_1/3Co_1/3O₂ Composite Positive Electrodes in All-Solid-State Lithium Batteries

“…474 cm ¹1 . 10,11 After the initial charge test, the intensity of the A 1g band decreased, which is originated from changes in local symmetry on the vibration mode. 10 In the charged NMC electrode, the E g band remained at ca.…”