Effects of charging rates on LiNi0.6Mn0.2Co0.2O2 (NMC622)/graphite Li-ion cells

doi:10.1016/j.jechem.2020.08.008

Journal of Energy Chemistry

Volume 56, May 2021, Pages 121-126

https://doi.org/10.1016/j.jechem.2020.08.008 Get rights and content

Abstract

Enabling fast charging capability of lithium-ion battery is of great importance to widespread adoption of electric vehicles. Increasing the charging rates from state-of-the-art 2C (30 min) to 6C (10 min) requires deep understanding on the cell aging mechanism. In this study, 400 mAh pouch cells are cycled at 1C, 4C and 6C charging rates with 1C discharging rate. Capacity fading, cathode structural changes, Li inventory loss, electrolyte composition changes and Li plating on graphite electrodes are thoroughly studied by various characterization techniques. The rapid capacity fading in cells at 6C charging rate is mainly due to Li inventory loss from cathode structure and metallic Li plating on graphite electrode at higher charging rate. Post-mortem analysis also revealed changes in electrolyte such as increased salt molarity and transesterification during fast charging.

Graphical abstract

Effects of different fast charging rates are studied on the capacity fading, electrode microstructural changes and electrolyte changes in LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622)/graphite pouch cells.

Introduction

The development of Li-ion cells with fast-charging capability is critical to further progress the widespread of electric vehicles [1]. The charging of an electric vehicle should ideally cost a similar amount of time compared to refueling a gasoline engine vehicle, which is 15 min or less. High-power Li-ion cells with thin electrodes are capable of recharging in short time (10~15 min). However, these cells have lower energy density and would lead to higher cell cost compared to state-of-the-art high energy density cells. A calculation by Ahmed et al. found a 10 min fast charging cell design with 19 µm thick anode increase the cell cost sharply to $196/kWh, compared to $107/kWh with 87 µm thick anode capable of 47 min charging [2]. In the high energy density Li-ion cells, fast charging can adversely lead to degradation in battery safety, energy density and cycle life.

Li plating has been identified as one of the most critical issues affecting battery performance [2], [3], [4], [5], [6], [7]. It occurs on the surface of the graphite electrode when large overpotentials in a cell decrease the local potential to below 0 V (vs. Li⁺/Li) [8], [9], [10]. Gallagher et al. have systematically studied the effect of electrode loading (thickness) and charging rate on Li plating [11]. It was found a relatively moderate charging rate of 1.5C (40 min charging) can have significant impact on Li-ion cells with 3.3 mAh/cm² loading, leading to capacity decrease and metallic Li deposition. Li plating can result in irreversible capacity loss due to the removal of active Li inventory in the cell [12]. Fully discharging of the cells with Li plating even at a low rate did not strip the deposited Li back to the positive electrode [11]. Another issue with Li plating is its high tendency to form dendrites with high surface area, which increase the parasitic reactions with electrolyte, forming isolated (dead) lithium, and thus reducing the Coulombic efficiency [13], [14], [15]. Li dendrite would also short the cell, cause catastrophic failure of the battery, and even inducing fatal safety hazards [16], [17], [18], [19].

Fast charging can also lead to rapid temperature rise from the high heat generation rate due to the large current applied to the cell. With embedded thermal couple in a 2-Ah pouch cell, Huang et al. observed the temperature of the cell rose from room temperature (23 °C) to 38 °C under 5C charging rate and 45 °C under 7C within 5 min [20]. This increased temperature can result in a decrease of the cell resistance to improve kinetics. Yang et al. have shown the cell performance during fast charging can be improved by intentionally heat the cell by internal heaters [21]. However, performance degradation can be aggravated by exposing the cell to elevated operation temperature. This is because parasitic reactions, like SEI growth, are intensified with increased temperatures [22]. Increase in the temperature can also worsen other unwanted reactions such as transition metal dissolution and binder decomposition [23], [24].

In the present study, we study the effects of fast charging (+1C, +4C and +6C) on the capacity fading, electrode microstructural changes and electrolyte changes in LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622)/graphite pouch cells. A detailed post-mortem analysis is performed to study the aging under different fast charging rates via electrochemical testing, neutron powder diffraction, inductively coupled plasma atomic emission spectroscopy (ICP-OES), gas chromatography–mass spectrometry (GC-MS), etc.

Section snippets

Experimental

Electrodes and pouch cells with capacity of 400 mAh were prepared in a dry room (dew point < −50 °C) at the DOE Battery Manufacturing R&D Facility (BMF) at Oak Ridge National Laboratory (ORNL). The cathode was NMC622 (Targray) electrodes with 2.3 mAh/cm² areal capacity loading and calendered to 30% porosity. The anode was graphite (Superior Graphite 1520T) electrodes with 2.6 mAh/cm² areal loading and calendered to 30% porosity. The electrolyte filled into the pouch cell was 1.2 M LiPF₆ in

Results and discussion

Fig. 1(a) shows the cycling performance of the NMC622/graphite pouch cell under different charging rates of 1C, 4C and 6C. The capacity shown on the Y-axis is based on the mass of the cathode materials NMC622. The capacity retentions after 200 cycles (compared the 1st cycle) are 94.0%, 89.4% and 73.8% under 1C, 4C and 6C charging rate, respectively. Fig. 1(b) shows the voltage curves of cells at the 1st, 50th and 200th cycles under different charging rates. During the 1st cycle, the capacities

Conclusions

Pouch cells with a moderate loading of 2.3 mAh/cm² were tested under different charging rates at 1C, 4C and 6C to evaluate its fast charging capabilities. Rapid capacity deterioration was observed for extreme fast charging (6C) cells with only 73.8% capacity retention after 200 cycles compared to 94.0% and 89.4% for 1C and 4C, respectively. An increasing portion of capacity for 6C was obtained from the high voltage (4.2 V) trickle charging. Post-mortem analysis on the cathode materials

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) (Technology Manager: Brian Cunningham). Neutron diffraction in this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. SEM was conducted at the

References (44)

S. Ahmed et al.
J. Power Sources
(2017)
T. Waldmann et al.
J. Power Sources
(2018)
C. Pastor-Fernández et al.
J. Power Sources
(2017)
J. Liu et al.
J. Power Sources
(2020)
Y. Gao et al.
J. Power Sources
(2017)
C. Uhlmann et al.
J. Power Sources
(2015)
S.S. Zhang et al.
J. Power Sources
(2006)
M. Broussely et al.
J. Power Sources
(2005)
Z. Li et al.
J. Power Sources
(2014)
X. Zhu et al.
J. Power Sources
(2020)

J. Vetter et al.

J. Power Sources

(2005)

T. Waldmann et al.

J. Power Sources

(2014)

S.S. Choi et al.

J. Power Sources

(2002)

K. Kumai et al.

J. Power Sources.

(1999)

H. Yoshida et al.

J. Power Sources

(1997)

H. Kim et al.

Electrochim. Acta

(2014)

Z. Du et al.

Electrochem. Commun.

(2019)

Z.A. Needell et al.

Nat. Energy

(2016)

Q. Liu et al.

RSC Adv.

(2016)

J.C. Burns et al.

J. Electrochem. Soc.

(2015)

K.G. Gallagher et al.

J. Electrochem. Soc.

(2016)

A.J. Smith et al.

J. Electrochem. Soc.

(2011)

Cited by (18)

Investigation of multi-step fast charging protocol and aging mechanism for commercial NMC/graphite lithium-ion batteries
2023, Journal of Energy Chemistry
Fast charging is considered a promising protocol for raising the charging efficiency of electric vehicles. However, high currents applied to Lithium-ion (Li-ion) batteries inevitably accelerate the degradation and shorten their lifetime. This work designs a multi-step fast-charging method to extend the lifetime of LiNi_0.5Co_0.2Mn_0.3O₂ (NMC)/graphite Li-ion batteries based on the studies of half cells and investigates the aging mechanisms for different charging methods. The degradation has been studied from both full cell behaviour and materials perspectives through a combination of non-destructive diagnostic methods and post-mortem analysis. In the proposed multi-step charging protocol, the state-of-charge (SOC) profile is subdivided into five ranges, and the charging current is set differently for different SOC ranges. One of the designed multi-step fast charging protocols is shown to allow for a 200 full equivalent cycles longer lifetime as compared to the standard charging method, while the charging time is reduced by 20%. From the incremental capacity analysis and electrical impedance spectroscopy, the loss of active materials and lithium inventory on the electrodes, as well as an increase in internal resistance for the designed multi-step constant-current-constant-voltage (MCCCV) protocol have been found to be significantly lower than for the standard charging method. Post-mortem analysis shows that cells aged by the designed MCCCV fast charging protocol exhibit less graphite exfoliation and crystallization damage, as well as a reduced solid electrolyte interphase (SEI) layer growth on the anode, leading to a lower R_sei resistance and extended lifetime.
Towards extreme fast charging of 4.6 V LiCoO<inf>2</inf> via mitigating high-voltage kinetic hindrance
2023, Journal of Energy Chemistry
Citation Excerpt :
Specifically, today at the lab level, increasing the cut-off voltage of LCO to 4.6 V can exploit its capacity up to 220 mAh g−1, which has been considered the maximum practical capacity of LCO [9–12]. Considering most of these devices would not last a day even under moderate use due to the limited storage capacity of LCO, extreme fast charging (XFC) provides an alternative solution to improve the usability and mobility of mobile electronics [13,14]. The high-rate characteristic of the high-voltage LCO critically depends on the fast transport of Li ions and electrons, for which the improvement of Li-ion diffusivity and electron conductivity is the key to success [15–18].
High-voltage LiCoO₂ (LCO) is an attractive cathode for ultra-high energy density lithium-ion batteries (LIBs) in the 3 C markets. However, the sluggish lithium-ion diffusion at high voltage significantly hampers its rate capability. Herein, combining experiments with density functional theory (DFT) calculations, we demonstrate that the kinetic limitations can be mitigated by a facial Mg²⁺+Gd³⁺ co-doping method. The as-prepared LCO shows significantly enhanced Li-ion diffusion mobility at high voltage, making more homogenous Li-ion de/intercalation at a high-rate charge/discharge process. The homogeneity enables the structural stability of LCO at a high-rate current density, inhibiting stress accumulation and irreversible phase transition. When used in combination with a Li metal anode, the doped LCO shows an extreme fast charging (XFC) capability, with a superior high capacity of 193.1 mAh g⁻¹ even at the current density of 20 C and high-rate capacity retention of 91.3% after 100 cycles at 5 C. This work provides a new insight to prepare XFC high-voltage LCO cathode materials.
Assessment and management of health status in full life cycle of echelon utilization for retired power lithium batteries
2022, Journal of Cleaner Production
Citation Excerpt :
Furthermore, thermal characteristics of the electrode material, electrolyte, and other components will change (Maher and Yazami, 2013), enthalpy change will decrease, entropy change will increase (Giel et al., 2018), and lithium deposition on the anode will significantly reduce the thermal stability of its material (Liu, J. et al., 2020; Ren et al., 2019; Zhang, L. et al., 2020). As the battery ages and decays in the macroscopic scale, lithium deposition on the anode becomes apparent (Lai et al., 2021c; Ren et al., 2019; Wu et al., 2021) (Fig. 5j), and the current collector becomes corroded electrochemically. Furthermore, mechanical stress may cause the foil to deform (Diao et al., 2021; Xie et al., 2020) (Fig. 5h), weakening the connection between the electrode and diaphragm, resulting in bulging and leaking (Mei et al., 2020) (Fig. 5f).
In recent years, a significant increase in market share has occurred in the global electric vehicle industry. These advancements will eventually lead to a large-scale decommissioning of power batteries. Global governments are actively promoting echelon utilization of retired power lithium batteries, enacting a series of policies and incentives, and the industry has also constructed large-scale projects in line with national strategies to extend the life cycle of retired power batteries. While echelon utilization for retired power lithium batteries is complex, it involves scientific assessment and management of battery health in the full life cycle. This paper summarizes relevant studies and technical progress and reconstructs a health assessment system for the full life cycle of echelon utilization for retired power lithium batteries. By extending the definition of battery health status and improving the assessment index of battery health status, the paper proposes an analysis method of health status in multi-scale, multi-dimensional, and multi-physical fields. We also analyze safety accident reports of energy storage plants, summarize the main factors that affect battery health, and propose a solution for integrated multi-stage and multi-level battery health management throughout the life cycle. Electricity and thermal management are core foundations and humidity, dust, and salt spray management for various geographical and climatic conditions. These technologies can be fully integrated with bionic principles in future studies. With the support of global governments, gradual implementation of supporting policies, and continuous advances in management and core technologies of battery health in the full life cycle of echelon utilization for retired lithium batteries, related industries will develop rapidly in the next five to ten years.
Effect of temperature on the high-rate pulse charging of lithium-ion batteries
2022, Journal of Electroanalytical Chemistry
Citation Excerpt :
However, due to the high active potential of the LTO anode, the energy density of the LTO batteries is only 67.5 % of the LiFePO4 batteries. The deterioration of cathode structure is another common aging mechanism in the process of high-rate charging, which mainly occurs in the lithium-ion batteries with LiNixCoyMnzO2 (NCM) or LiNixCoyAlzO2 (NCA) blend cathodes [19]. S. Zhang et al. studied the aging mechanism of lithium batteries during rapid charging at 10C.
Based on the residual energy recovery in the electromagnetic emission scenario, the 30C pulse charging cycle experiments of LiFePO₄ batteries customized for electromagnetic emission at different charging temperatures were carried out to study the influence of charging temperature on battery aging. By adjusting the ambient temperature, heat dissipation conditions, and rest time, we studied the battery aging process at the average charging temperatures of 16 °C, 21 °C, 26 °C, 30 °C and 35 °C. Experimental results show that increasing charging temperature can significantly delay battery aging and prolong battery cycle life. In addition, when the average charging temperature is lower than 30 °C, the battery shows nonlinear aging; when the average charging temperature is higher than 30 °C, the battery shows linear aging in the early stage of cycling and nonlinear aging at the end of the cycle. The results of differential capacity analysis (DCA) show that the loss of lithium inventory is the primary aging mode of the battery and did not change with charging temperature. The aging mechanism of the battery under high-rate pulse charging was studied through multiscale post-test analysis. Analysis results showed that with the increase of charging temperature, the area of lithium-plating area decreases, and the thickness of SEI film increases. It can be inferred that when the average charging temperature is lower than 30 °C, lithium plating is the primary aging mechanism of the LiFePO₄ battery customized for electromagnetic emission. When the average charging temperature is higher than 30 °C, the growth of SEI film is the primary aging mechanism of the battery.
A robust in-situ catalytic graphitization combined with salt-template strategy towards fast lithium-ions storage
2022, Journal of Alloys and Compounds
Lithium-ion hybrid supercapacitors (LIHSs) have received significant attention in electrochemical energy-related areas. However, the sluggish anode kinetics hinders the improvement of power density and cycling stability. Herein, an in-situ catalytic graphitization with a salt-template strategy was applied to prepare the porous graphitic carbon nanosheets. The results demonstrated that the addition of KCl led to a higher specific surface area and higher porous structure. The as-prepared potassium-induced porous graphitic carbon flakes (PPGCs) exhibited an electrochemical plateau capacity of 240.1 mAh g⁻¹ and an electrochemical slope capacity of 129.0 mAh g⁻¹. PPGCs also delivered superior rate performance of 60.8 mAh g⁻¹ even at 5.0 A g⁻¹, which was much better than graphitic carbon flakes (GCFs) and potassium-induced graphitic carbon flakes (PGCFs). When assembled into LIHSs devices with commercial activated carbon, the energy density reached 75.6 Wh kg⁻¹, and the power density was as high as 5.05 kW kg⁻¹. After 10,000 cycles at 5.0 A g⁻¹, the energy density was maintained at 39.2 Wh kg⁻¹, revealing excellent long-term cycling performance. This work may provide a promising solution to obtain graphitic carbon with a high graphitization degree and highly porous structure.
The fast-charging properties of micro lithium-ion batteries for smart devices
2022, Journal of Colloid and Interface Science
Citation Excerpt :
Lithium plating is considered to be the main factor limiting the fast charging of LIBs due to the relationship between lithium plating and other degradation mechanisms [11]. Studies on fast charging based on commercial LIBs are more about exploring the impacts of charging protocols, low temperature on the batteries and the mechanism of their performance degradation under fast charging conditions in the past few years [10,12–15]. It is worth noting that most of these studies based on commercial LIBs are about high energy batteries used for EVs.
Nowadays fast charging has become an important characteristic of lithium-ion batteries (LIBs), so is of great significance to study the fast charging of LIBs. However, previous research of fast charging has focused more on high energy density LIBs, due to the growing demand for electric vehicles. Herein, the fast-charging properties under ambient temperature and high temperature for (60 mAh LiCoO₂/graphite batteries) micro-LIBs are firstly investigated. The electrochemical test results reveal that this kind of battery possesses 4C fast-charging capability. Further increase in charging rate will accelerate battery capacity decay without reducing charging time. Although high temperature increases the fast-charging capacity and shortens the fast-charging time to 10 min at 6C under 65 °C, increase of side reactions resulted from high temperature also exacerbates the performance of battery. Post-mortem analysis further demonstrates the structural changes of cathode and anode materials, residual lithium deposits, peeling of graphite and the incrassation of the solid electrolyte interphase (SEI), especially under high temperature, which lead to fast -charging performance degradation. This work reveals the possible causes of micro battery performance deterioration during fast charging under ambient and high temperature and provides some reference for designing micro-LIBs with fast -charging properties.

View all citing articles on Scopus

View full text