Advances in acoustic techniques for evaluating defects and properties in lithium-ion batteries: A review

With the rapid demand for high-performance energy storage systems, lithium-ion batteries (LiBs) have emerged as the predominant technology in various applications. However, ensuring the safety and reliability of these batteries remains a critical challenge. Ultrasound-based detection, as a non-destructive and effective method for monitoring the internal state of LiBs, has gradually emerged as a valuable tool to enhance battery safety, reliability, and performance. This paper provides a review of recent advancements in the field of acoustic detection for LiBs, delving into the fundamental principles and mechanisms governing the propagation of acoustic signals within these batteries. This paper reviews the correlation between these acoustic signals and the operational status of the battery, as well as the association with internal side reactions during abnormal conditions. The strengths and limitations of current ultrasound-based detection methods are emphasized, offering insights to guide researchers, engineers, and industry professionals in advancing the field. The review aims to foster the development of robust ultrasound-based detection solutions for the next generation of energy storage systems.

Impact of Individual Cell Parameter Difference on the Performance of Series-Parallel Battery Packs

Lithium-ion power batteries are used in groups of series-parallel configurations. There are Ohmic resistance discrepancies, capacity disparities, and polarization differences between individual cells during discharge, preventing a single cell from reaching the lower limit of the terminal voltage simultaneously, resulting in low capacity and energy utilization. The effect of the parameter difference (difference in parameters) of individual cells on the performance of the series-parallel battery pack is simulated and analyzed by grouping cells with different parameters. The findings reveal that when cells are connected in series, the capacity difference is a significant factor impacting the battery pack's energy index, and the capacity difference and Ohmic resistance difference are significant variables affecting the battery pack's power index. When cells are connected in parallel, the difference in Ohmic internal resistance between them causes branch current imbalance, low energy utilization in some individual cells, and a sharp expansion of unbalanced current at the end of discharge, which is prone to overdischarge and shortens battery life. Interestingly, we found that when there is an aging cell in a series-parallel battery pack, the terminal voltage of the single battery module containing the aging single cell will decrease sharply at the end of discharge. Evaluating the change rate of battery module terminal voltage at the end of discharge can be used as a method to evaluate the aging degree of the battery module. The research results provide a reference for connecting batteries to battery packs, particularly the screening of retired power battery packs and the way to reconnect into battery packs.

Ni-Ion-Chelating Strategy for Mitigating the Deterioration of Li-Ion Batteries with Nickel-Rich Cathodes

Ni-rich cathodes are the most promising candidates for realizing high-energy-density Li-ion batteries. However, the high-valence Ni⁴⁺ ions formed in highly delithiated states are prone to reduction to lower valence states, such as Ni³⁺ and Ni²⁺ , which may cause lattice oxygen loss, cation mixing, and Ni ion dissolution. Further, LiPF₆ , a key salt in commercialized electrolytes, undergoes hydrolysis to produce acidic compounds, which accelerate Ni-ion dissolution and the interfacial deterioration of the Ni-rich cathode. Dissolved Ni ions migrate and deposit on the surface of the graphite anode, causing continuous electrolyte decomposition and threatening battery safety by forming Li dendrites on the anode. Herein, 1,2-bis(diphenylphosphino)ethane (DPPE) chelates Ni ions dissolved from the Ni-rich cathode using bidentate phosphine moieties and alleviates LiPF₆ hydrolysis via complexation with PF₅ . Further, DPPE reduces the generation of corrosive HF and HPO₂ F₂ substantially compared to the amounts observed using trimethyl phosphite and tris(trimethylsilyl) phosphite, which are HF-scavenging additives. Li-ion cells with Ni-rich cathodes and graphite anodes containing DPPE exhibit remarkable discharge capacity retentions of 83.4%, with high Coulombic efficiencies of >99.99% after 300 cycles at 45 °C. The results of this study will promote the development of electrolyte additives.

Effect of High-Rate Cycle Aging and Over-Discharge on NCM811 (LiNi0.8Co0.1Mn0.1O2) Batteries

Inconsistencies in a monomer battery pack can lead to the over-discharge of a single battery. Although deep over-discharge can be avoided by optimizing the battery control system, slight over-discharge still often occurs in the battery pack. The aging behavior of cylindrical NCM811 batteries under high-rate aging and over-discharge was studied. By setting the end-of-discharge of 1 V, the battery capacity rapidly decayed after 130 cycles. Additionally, the temperature sharply increased in the over-discharge stage. The micro short-circuit was found by the discharge voltage curve and impedance spectrum. Batteries with 100%, 79.6% and 50.9% SOH (state of health = Q_now/Q_new × 100%) as a result of high-rate aging and over-discharging were subjected to thermal testing in an adiabatic environment. The battery without high-rate aging and over-discharge did not experience thermal runaway. However, severe thermal runaway occurred in the 79.6% and 50.9% SOH batteries. Regarding the cyclic aging of the 50.9% SOH battery, the fusion temperature of the separator decreased by 22.3 °C, indicating a substantial degradation of the separator and thus reducing battery safety. Moreover, the results of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses revealed that the particles of the positive material were broken and detached, and that large-area cracks and delamination had formed on the negative material. Furthermore, Ni deposition and the uneven deposition of P and F on the negative surface were observed, which increased the risk of short-circuit in the battery. Positive and negative materials were attached on both sides of the separator, which reduced the effective area of ionic transportation.

Aging Behavior of Lithium Titanate Battery under High-Rate Discharging Cycle

The high-rate discharging performance of a lithium titanate battery is one of its main properties. In conditions that require ultra-high-rate discharging, a lithium titanate battery can be discharged continuously at a current of 50 C (50 times of its maximum capacity) or higher. In this paper, we take cylindrical steel shell lithium titanate cells as the research object and perform aging cycles at 66 C on these cells. The ultra-high-rate discharging cycles cause a rapid high-power capacity fading while the available capacity at normal current rate is not affected. The capacity at 66 C decreases to 80% of initial value in 10 cycles. This paper also analyzes the aging process of a lithium titanate battery at high-rate discharging with incremental capacity (IC) analysis, and presents the aging behavior of lithium titanate battery qualitatively, which is inconsistent with existing research. We attribute the aging mechanism of ultra-high-rate discharging cycles to the decrease of ionic mobility and increase of polarization resistance. Mechanical damage is observed in the CT scan of an aged cell, which we presume to be the result of rapid strain of cathode material.

Comprehensive Investigation of a Slight Overcharge on Degradation and Thermal Runaway Behavior of Lithium-Ion Batteries

Overcharge is a hazardous abuse condition that has dominant influences on cell performance and safety. This work, for the first time, comprehensively investigates the impact of different overcharge degrees on degradation and thermal runaway behavior of lithium-ion batteries. The results indicate that single overcharge has little influence on cell capacity, while it severely degrades thermal stability. Degradation mechanisms are investigated by utilizing the incremental capacity-differential voltage and relaxation voltage analyses. During the slight overcharge process, the conductivity loss and the loss of lithium inventory always occur; the loss of active material starts happening only when the cell is overcharged to a certain degree. Lithium plating is the major cause for the loss of lithium inventory, and an interesting phenomenon that the arrival time of the dV/dt peak decreases linearly with the increase of the overcharge degree is found. The cells with different degrees of overcharge exhibit a similar behavior during adiabatic thermal runaway. Meanwhile, the relationship between sudden voltage drop and thermal runaway is further established. More importantly, the characteristic temperature of thermal runaway, especially the self-heating temperature (T₁), decreases severely along with overcharging, which means that a slight overcharge severely decreases the cell thermal stability. Further, post-mortem analysis is conducted to investigate the degradation mechanisms. The mechanism of the side reactions caused by a slight overcharge on the degradation performance and thermal runaway characteristics is revealed.

Unanticipated Mechanism of the Trimethylsilyl Motif in Electrolyte Additives on Nickel-Rich Cathodes in Lithium-Ion Batteries

The introduction of a trimethylsilyl (TMS) motif in electrolyte additives for lithium-ion batteries is regarded as an effectual approach to remove corrosive hydrofluoric acid (HF) that structurally and compositionally damages the electrode-electrolyte interface and gives rise to transition metal dissolution from the cathode. Herein, we present that electrolyte additives with TMS moieties lead to continued capacity loss of polycrystalline (PC)-LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathodes coupled with graphite anodes compared to additives without TMS as the cycle progresses. Through a comparative study using electrolyte additives with and without TMS moieties, it is revealed that the TMS group is prone to react with residual lithium compounds, in particular, lithium hydroxide (LiOH) on the PC-NCM811 cathode, and the resulting TMS-OH triggers the decomposition of PF₅ created by the autocatalytic decomposition of LiPF₆ that generates reactive species, namely, HF and POF₃. This work aims to offer a way to build favorable interface structures for Ni-rich cathodes covered with residual lithium compounds through a study to figure out the roles of TMS moieties of electrolyte additives.

Determining phase transitions of layered oxides via electrochemical and crystallographic analysis

The chemical diffusion coefficient in LiNi_1/3Mn_1/3Co_1/3O₂ was determined via the galvanostatic intermittent titration technique in the voltage range 3 to 4.2 V. Calculated diffusion coefficients in these layered oxide cathodes during charging and discharging reach a minimum at the open-circuit voltage of 3.8 V and 3.7 V vs. Li/Li⁺, respectively. The observed minima of the chemical diffusion coefficients indicate a phase transition in this voltage range. The unit cell parameters of LiNi_1/3Mn_1/3Co_1/3O₂ cathodes were determined at different lithiation states using ex situ crystallographic analysis. It was shown that the unit cell parameter variation correlates well with the observed values for chemical diffusion in NMC cathodes; with a notable change in absolute values in the same voltage range. We relate the observed variation in unit cell parameters to the nickel conversion into the trivalent state, which is Jahn-Teller active, and to the re-arrangement of lithium ions and vacancies.

Effects of Overdischarge Rate on Thermal Runaway of NCM811 Li-Ion Batteries

Overdischarge often occurs during the use of battery packs, owing to cell inconsistency in the pack. In this study, the overdischarge behavior of 2.9 Ah cylindrical NCM811 [Li(Ni0.8Co0.1Mn0.1)O2] batteries in an adiabatic environment was investigated. A higher overdischarge rate resulted in a faster temperature increase in the batteries. Moreover, the following temperatures increased: Tu, at which the voltage decreased to 0 V; Ti, at which the current decreased to 0 A; and the maximum temperature during the battery overdischarge (Tm). The following times decreased: tu, when the voltage decreased from 3 to 0 V, and ti, when the current decreased to 0 A. The discharge capacity of the batteries was 3.06–3.14 Ah, and the maximum discharge depth of the batteries was 105.51–108.27%. Additionally, the characteristic overdischarge behavior of the batteries in a high-temperature environment (55 °C) was investigated. At high temperatures, the safety during overdischarging decreased, and the amount of energy released during the overdischarge phase and short-circuiting decreased significantly. Shallow overdischarging did not significantly affect the battery capacity recovery. None of the overdischarging cases caused fires, explosions, or thermal runaway in the batteries. The NCM811 batteries achieved good safety performance under overdischarge conditions: hence, they are valuable references for battery safety research.

Accelerated aging and degradation mechanism of LiFePO4/graphite batteries cycled at high discharge rates

The effects of discharge rates (0.5C, 1.0C, 2.0C, 3.0C, 4.0C and 5.0C) on the aging of LiFePO4/graphite full cells are researched by disassembling the fresh and aged full cells. The capacity degradation mechanism is analyzed via electrochemical performance, surface morphologies and compositions, and the structure of the anode and cathode electrodes. The capacity fade is accelerated with increasing discharge rates. The irreversible loss of active lithium due to the generation of an SEI film is the primary aging factor for the full cells cycled at low discharge rates. However, when the discharge rate is greater than or equal to 4.0C, the performance degradation of the LiFePO4 electrode is distinct due to structure decay, which is caused by quick and repeated intercalation of lithium ions and elevated temperature during discharging. In addition, the SEI film on the anode tends to be unstable after the rapid extraction of lithium ions at high discharge rates, and this enhances the loss of active lithium. Therefore, it is indicated that the degradation mechanism is changed for the full cells aged at 4.0C and 5.0C. Besides, the high discharge rate also increases the internal resistance of the full cell, which is detrimental to high rate discharge performance.

Investigation of a commercial lithium-ion battery under overcharge/over-discharge failure conditions

A lithium-ion battery (LIB) may experience overcharge or over-discharge when it is used in a battery pack because of capacity variation of different batteries in the pack and the difficulty of maintaining identical state of charge (SOC) of every single battery.