Yiqing Huang

Thermal Stability and Reactivity of Cathode Materials for Li-Ion Batteries

The thermal stability of electrochemically delithiated Li0.1Ni0.8Co0.15Al0.05O2 (NCA), FePO4 (FP), Mn0.8Fe0.2PO4 (MFP), hydrothermally synthesized VOPO4, LiVOPO4, and electrochemically lithiated Li2VOPO4 is investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis, coupled with mass spectrometry (TGA-MS). The thermal stability of the delithiated materials is found to be in the order of NCA < VOPO4 < MFP < FP. Unlike the layered oxides and MFP, VOPO4 does not evolve O2 on heating. Thus, VOPO4 is less likely to cause a thermal run-away phenomenon in batteries at elevated temperature and so is inherently safer. The lithiated materials LiVOPO4, Li2VOPO4, and LiNi0.8Co0.15Al0.05O2 are found to be stable in the presence of electrolyte, but sealed-capsule high-pressure experiments show a phase transformation of VOPO4 → HVOPO4 → H2VOPO4 when VOPO4 reacts with electrolyte (1 M LiPF6 in EC/DMC = 1:1) between 200 and 300 °C. Using first-principles calculations, we confirm that the charged VOPO4 cathode is indeed predicted to be marginally less stable than FP but significantly more stable than NCA in the absence of electrolyte. An analysis of the reaction equilibria between VOPO4 and EC using a multicomponent phase diagram approach yields products and reaction enthalpies that are highly consistent with the experiment results.

TUNING THE ACTIVITY OF OXYGEN IN LINI0.8CO0.15AL0.05O2 BATTERY ELECTRODES

Layered transition metal oxides such as LiNi0.8Co 0.15Al0.05O2 (NCA) are highly desirable battery electrodes. However, these materials suffer from thermal runaway caused by deleterious oxygen loss and surface phase transitions when in highly overcharged and overheated conditions, prompting serious safety concerns. Using in situ environmental transmission electron microscopy techniques, we demonstrate that surface oxygen loss and structural changes in the highly overcharged NCA particles are suppressed by exposing them to an oxygen-rich environment. The onset temperature for the loss of oxygen from the electrode particle is delayed to 350 °C at oxygen gas overpressure of 400 mTorr. Similar heating of the particles in a reducing hydrogen gas demonstrated a quick onset of oxygen loss at 150 °C and rapid surface degradation of the particles. The results reported here illustrate the fundamental mechanism governing the failure processes of electrode particles and highlight possible strategies to circumvent such issues.

Understanding the stability of MnPO4

We have revealed the critical role of carbon coating in the stability and thermal behaviour of olivine MnPO4 obtained by chemical delithiation of LiMnPO4. (Li)MnPO4 samples with various particle sizes and carbon contents were studied. Carbon-free LiMnPO4 obtained by solid state synthesis in O2 becomes amorphous upon delithiation. Small amounts of carbon (0.3 wt%) help to stabilize the olivine structure, so that completely delithiated crystalline olivine MnPO4 can be obtained. Larger amount of carbon (2 wt%) prevents full delithiation. Heating in air, O2, or N2 results in structural disorder (<300 °C), formation of an intermediate sarcopside Mn3(PO4)2 phase (350–450 °C), and complete decomposition to Mn2P2O7 on extended heating at 400 °C. Carbon coating protects MnPO4 from reacting with environmental water, which is detrimental to its structural stability.

What Happens to LiMnPO4 upon Chemical Delithiation

Olivine MnPO4 is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO4, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO4. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO4, which causes amorphization of olivine MnPO4. The properties of crystalline MnPO4 obtained from carbon-coated LiMnPO4 and of the amorphous product resulting from delithiation of pure LiMnPO4 were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP2O7 and MnH2P2O7 from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO4 is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal.

Correlating Lithium Hydroxyl Accumulation with Capacity Retention in V2O5 Aerogel Cathodes

V2O5 aerogels are capable of reversibly intercalating more than 5 Li(+)/V2O5 but suffer from lifetime issues due to their poor capacity retention upon cycling. We employed a range of material characterization and electrochemical techniques along with atomic pair distribution function, X-ray photoelectron spectroscopy, and density functional theory to determine the origin of the capacity fading in V2O5 aerogel cathodes. In addition to the expected vanadium redox due to intercalation, we observed LiOH species that formed upon discharge and were only partially removed after charging, resulting in an accumulation of electrochemically inactive LiOH over each cycle. Our results indicate that the tightly bound water that is necessary for maintaining the aerogel structure is also inherently responsible for the capacity fade.

Electrochemical Performance of Nanosized Disordered LiVOPO4

ε-LiVOPO4 is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO4, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by 7Li and 31P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO4 at C/50 (1 C = 153 mA h g–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO4/Li3VO4/acetylene black composite yields the high cycling capacity of 250 mA h g–1 at C/5 for over 70 cycles.

In-Situ TEM Study of Phase Evolution in Individual Battery Materials

There has been significant interest in understanding the mechanism of structural phase evolution occurring in individual battery materials at different state of charge (SOC) levels and at various environmental constraints [1, 2]. For example, a commercially-important LiNi0.8Co0.15Al0.05O2 (NCA) cathode material when over-discharged (> 4.2 V) and over-heated can lead to the loss of stoichiometric oxygen from the surface [1, 2]. The loss of oxygen is detrimental as it can react with inflammable liquid electrolyte and cause thermal runaway. Furthermore, the loss of oxygen is accompanied by the migration/re-ordering of transition metal (TM) ions, which leads to complex phase transformation: (R3̅m) → disordered spinel (Fd3̅m) → disordered rock salt (Fm3̅m). The spinel/rock-salt phase that forms on the surface increases the impedance and degrades the electrochemical activity of the electrode. The local probing of the structural and chemical changes that occur within the individual battery material is thus important. Conventional X-ray techniques are insensitive to localized phase transformation, as they provide only average information from ensemble of particles. In-situ environmental transmission electron microscopy (ETEM) provides a unique platform where individual nanoparticles can be investigated for any morphological, structural or chemical changes, under external stimuli, in real-time [3]. Furthermore, the aberration-corrected ETEM with a differential pumping apparatus allows high spatial resolution of < 0.1 nm even in a high-pressure gas environment (e.g., O2, H2) in the system.

The Intermediate State of the Layered → Spinel Phase Transformation in LiNi0.80Co0.15Al0.05O2 Cathode

Layered LiNi0.80Co0.15Al0.05O2 (NCA) is a promising cathode material for lithium ion batteries (LIBs), which has a high rate capability, a long lifetime and theoretically a high specific capacity. The aluminum addition prevents the NCA layered structure from collapsing into an inactive rock-salt phase, but it also accelerates the spinel phase transformation. The spinel phase formed in the surface region increases the impedance of NCA, reduces the electrochemical activity and diminishes the overall capacity.

ETEM Study of Oxygen Activity in LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) Cathode Materials at Various States of Charge

Lithium ion batteries (LIBs) have been predominantly used in the consumer electronics and other power devices. The drive to use LIBs in the large-scale applications such as electric vehicles (EVs) and smart grids has spurred significant research activity, particularly concerning the cathode electrodes. The current cathode materials, LiCoO2 (~140 mAh/g) or LiFePO4 (~160 mAh/g), have relatively low energy densities, and can hardly match with the capacities of the next generation of anode materials (e.g. Si, ~4200 mAh/g). To improve upon this limitation, layered materials such as LiNi0.8Co0.15Al0.05O2 (NCA) & LiNixMn1-x-yCoyO2 (NCM) – which have discharge capacity ~200 mAh/g – have been actively pursed as potential replacements. However, these cathode materials suffer from rapid capacity fade and poor thermal instability, thus raising serious safety concerns. For example, these materials in a highly delithiated state (overcharged) can readily release oxygen at high temperature, and lead to complex phase transitions: layered (R-3m)  disordered spinel (Fd-3m)  rock-salt (Fm-3m). The released O2 can react with the flammable electrolyte, leading to thermal-runaway and catastrophic battery failure. Therefore, it is critical to understand the role that the oxygen release plays in the migration of transition metal (TM) cations (Ni, Co, & Mn) during the various phase transition processes. Environmental transmission electron microscopy (ETEM) provides a unique platform where individual nanoparticles can be investigated for any morphological, structural or chemical changes, under external stimuli, in real-time. Furthermore, the aberration-corrected ETEM with a differential pumping apparatus allows high spatial resolution of < 0.1 nm even in a high-pressure gas environment (e.g., O2, H2) in the system. Here, we use in-situ ETEM to understand the role that oxygen plays in the rearrangement of the TM ions both at the surface & in the bulk of the NCA materials at elevated temperatures.