Vibration durability testing of nickel manganese cobalt oxide (NMC) lithium-Ion 18,650 battery cells

Electric vehicle (EV) manufacturers are employing cylindrical format cells in the construction of the vehicles’ battery systems. There is evidence to suggest that both the academic and industrial communities have evaluated cell degradation due to vibration and other forms of mechanical loading. The primary motivation is often the need to satisfy the minimum requirements for safety certification. However, there is limited research that quantifies the durability of the battery and in particular, how the cells will be affected by vibration that is representative of a typical automotive service life (e.g., 100,000 miles). This paper presents a study to determine the durability of commercially available 18,650 cells and quantifies both the electrical and mechanical vibration-induced degradation through measuring changes in cell capacity, impedance and natural frequency. The impact of the cell state of charge (SOC) and in-pack orientation is also evaluated. Experimental results are presented which clearly show that the performance of 18,650 cells can be affected by vibration profiles which are representative of a typical vehicle life. Consequently, it is recommended that EV manufacturers undertake vibration testing, as part of their technology selection and development activities to enhance the quality of EVs and to minimize the risk of in-service warranty claims

Battery cycle life test development for high-performance electric vehicle applications

High Performance (HP) battery electric vehicle (BEV) and racing applications represent significantly different use cases than those associated with conventional consumer vehicles and road driving. The differences between HP use cases and the duty-cycles embodied within established battery test standards will lead to unrepresentative estimates for battery life and performance within a HP application. A strategic requirement exists to define a methodology that may be used to create a representative HP duty-cycle. Within this paper two methods HP duty-cycle design are evaluated and validated. Extensive simulation results into the electrical performance and heat generation within the battery highlight that the new HP duty-cycles provide a more representative duty-cycle compared to traditional battery test standards. The ability to more accurately predict the performance requirements for the battery system within this emerging and strategically important BEV sector will support a range of engineering functions. In addition, the ability to more accurately define the use-case for a HP-BEV will underpin ongoing experimentation and mathematical modelling to quantify the associated cell ageing and degradation that may occur within HP vehicle applications

Analysis of a battery management system (BMS) control strategy for vibration aged nickel manganese cobalt oxide (NMC) Lithium-Ion 18650 battery cells

Electric vehicle (EV) manufacturers are using cylindrical format cells as part of the vehicle’s rechargeable energy storage system (RESS). In a recent study focused at determining the ageing behavior of 2.2 Ah Nickel Manganese Cobalt Oxide (NMC) Lithium-Ion 18650 battery cells, significant increases in the ohmic resistance (RO) were observed post vibration testing. Typically a reduction in capacity was also noted. The vibration was representative of an automotive service life of 100,000 miles of European and North American customer operation. This paper presents a study which defines the effect that the change in electrical properties of vibration aged 18650 NMC cells can have on the control strategy employed by the battery management system (BMS) of a hybrid electric vehicle (HEV). It also proposes various cell balancing strategies to manage these changes in electrical properties. Subsequently this study recommends that EV manufacturers conduct vibration testing as part of their cell selection and development activities so that electrical ageing characteristics associated with road induced vibration phenomena are incorporated to ensure effective BMS and RESS performance throughout the life of the vehicle

Identification and quantification of ageing mechanisms in Lithium-ion batteries using the EIS technique

Ageing diagnosis in Lithium-ion batteries is essential to ensure their reliability and optimum performance over time. The Battery Management System (BMS) usually monitors battery ageing with the aid of two metrics: capacity and power fade. However, these metrics do not identify the main root causes of battery ageing. Using the Electrochemical Impedance Spectroscopy technique, this work proposes a novel method to identify and quantify ageing mechanisms over time. The method is applied to four parallelised Lithium-ion cells cycled with a constant driving profile for 500 cycles. As a result, Loss of Active Material (LAM) and Loss of Lithium Ions (LLI) were found to be the most pertinent ageing mechanisms over time for the four cells. Identification and quantification of ageing mechanisms will support novel battery lifetime control strategies within the BMS, so that potential failures during normal operation are prevented

On the possibility of extending the lifetime of lithium-ion batteries through optimal V2G facilitated by a flexible integrated vehicle and smart-grid system

Renewable energies are a key pillar of power sector decarbonisation. Due to the variability and uncertainty they add however, there is an increased need for energy storage. This adds additional infrastructure costs to a degree that is unviable: for an optimal case of 15GW of storage by 2030, the cost of storage is circa: £1000/kW. A promising solution to this problem is to use the batteries contained within electric vehicles (EVs) equipped with bi-directional charging systems to facilitate ancillary services such as frequency regulation and load balancing through vehicle to grid (V2G) technologies. Some authors have however dismissed V2G as economically unviable claiming the cost of battery degradation is larger than arbitrage. To thoroughly address the viability of V2G technologies, in this work we develop a comprehensive battery degradation model based on long-term ageing data collected from more than fifty long-term degradation experiments on commercial C6/LiNiCoAlO2 batteries. The comprehensive model accounts for all established modes of degradation including calendar age, capacity throughput, temperature, state of charge, depth of discharge and current rate. The model is validated using six operationally diverse real-world usage cycles and shows an average maximum transient error of 4.6% in capacity loss estimates and 5.1% in resistance rise estimates for over a year of cycling. This validated, comprehensive battery ageing model has been integrated into a smart grid algorithm that is designed to minimise battery degradation. We show that an EV connected to this smart-grid system can accommodate the demand of the power network with an increased share of clean renewable energy, but more profoundly that the smart grid is able to extend the life of the EV battery beyond the case in which there is no V2G. Extensive simulation results indicate that if a daily drive cycle consumes between 21% and 38% state of charge, then discharging 40% to 8% of the batteries state of charge to the grid can reduce capacity fade by approximately 6% and power fade by 3% over a three month period. The smart-grid optimisation was used to investigate a case study of the electricity demand for a representative University office building. Results suggest that the smart-grid formulation is able to reduce the EVs’ battery pack capacity fade by up to 9.1% and power fade by up to 12.1%

Vibration durability testing of Nickel Cobalt Aluminum Oxide (NCA) lithium-ion 18650 battery cells

This paper outlines a study undertaken to determine if the electrical performance of Nickel Cobalt Aluminum Oxide (NCA) 3.1 Ah 18650 battery cells can be degraded by road induced vibration typical of an electric vehicle (EV) application. This study investigates if a particular cell orientation within the battery assembly can result in different levels of cell degradation. The 18650 cells were evaluated in accordance with Society of Automotive Engineers (SAE) J2380 standard. This vibration test is synthesized to represent 100,000 miles of North American customer operation at the 90th percentile. This study identified that both the electrical performance and the mechanical properties of the NCA lithium-ion cells were relatively unaffected when exposed to vibration energy that is commensurate with a typical vehicle life. Minor changes observed in the cell’s electrical characteristics were deemed not to be statistically significant and more likely attributable to laboratory conditions during cell testing and storage. The same conclusion was found, irrespective of cell orientation during the test

Preparation and characterisation of Pt/C and Ni/C modified electrocatalysts for use towards the oxygen reduction reaction for proton exchange membrane fuel cells

The aim of this thesis is to develop more active catalysts for the oxygen reduction reaction whilst decreasing the metal content to drive forward the emergence of the fuel cell technology on the market. Chapter 3 presents the preparation of Ni modified Pt/C catalysts (Ni(acac)2 and Ni(Cp)2) using the controlled surface modification technique. The resulting catalysts were heat treated at 200, 500, 750 and 900 °C and the catalysts were characterised by ICP-OES, TEM, EDX, CV, RDE, EXAFS and XPS. The catalysts exhibited up to 8-fold increase in specific activity and up to 9-fold increase in mass activity. The increase in activity was assigned to (a) the synergistic effect of Ni on Pt and (b) the degree of alloying which has two consequences: (a) decrease of the Pt d-band centre and (b) change of the arrangement of the Pt and Ni atoms at the surface of the particles. The decrease in the Pt d-band centre resulted in the lowering of the adsorption strength of the oxide species which in turn led to a lower Pt-O coverage. This was supported by the decrease of the reduction potential of the oxide reduction peak as the heat treatment temperature increased. In addition, as the heat treatment temperature increased, the Pt surface concentration increased due to the diffusion of the Ni atoms inwards and the diffusion of the Pt atoms towards the outside of the particle. This led to larger and more well-defined Pt crystal planes. The presence of more well-defined Pt crystal planes seemed to provide more suitable adsorption site for the dual-site adsorption of the oxygen, thus increasing the activity. Last but not least, the highest increase in catalytic activity was exhibited by the catalysts heat treated at 500 °C. This demonstrated the importance of the choice of the secondary metal and the importance of the arrangement of the atoms at the surface of the particles. Chapter 4 presents what is believed to be the first attempt to prepare Pt modified Ni/C catalysts (Pt(acac)2) using the controlled surface modification technique. The catalysts were characterised by TEM, CV and RDE. The deposition of the Pt precursor was shown to be incomplete; however, the catalysts still had a Pt content of ~ 4 wt%. Despite the low Pt content, the catalysts exhibited up to 8-fold increase in specific activity. The increase in activity was assigned to the synergistic effect between Ni and Pt which was shown by the decrease in the lattice parameter and the decrease of the overpotential of the oxide reduction peak. Chapter 5 offers a summary of the thesis as well as a list of the strategies employed to date to increase the catalytic activity of the cathode catalysts. It also includes some suggestion for future work including underpotential deposition (UPD), MEA testing and stability testin

Multi-axis vibration durability testing of lithium ion 18650 NCA cylindrical cells

This paper presents new research to determine if the electromechanical attributes of Nickel Cobalt Aluminium Oxide (NCA) 18650 battery cells are adversely affected by exposure to vibration commensurate with that experienced by electric vehicles (EVs) through road induced excitation. This investigation applied vibration to a set of commercially available cells in six degrees of freedom (6-DOF) using a multi-axis shaker table. This method of mechanical testing is known to be more representative of the vibration experienced by automotive components, as 6 motions of vibration (X, Y, Z, roll, pitch and yaw) are applied simultaneously. Within the context of this study, cell characterisation within the electrical domain is performed via quantification of the cell’s impedance, the open-circuit potential and the cell’s energy capacity. Conversely, the mechanical properties of the cell are inferred through measurement of the cell’s natural frequency. Experimental results are presented that highlight that the electromechanical performances of the 18650 NCA cells do not, in the main, display statistically significant degradation when subject to vibration representative of a typical 10-year European vehicle life. However, a statistically significant increase in DC resistance of the cells was observed

Data for Battery cycle life test development for high-performance electric vehicle applications

High Performance (HP) battery electric vehicle (BEV) and racing applications represent significantly different use cases than those associated with conventional consumer vehicles and road driving. The differences between HP use cases and the duty-cycles embodied within established battery test standards will lead to unrepresentative estimates for battery life and performance within a HP application. A strategic requirement exists to define a methodology that may be used to create a representative HP duty-cycle. Within this paper two methods HP duty-cycle design are evaluated and validated. Extensive simulation results into the electrical performance and heat generation within the battery highlight that the new HP duty-cycles provide a more representative duty-cycle compared to traditional battery test standards. The ability to more accurately predict the performance requirements for the battery system within this emerging and strategically important BEV sector will support a range of engineering functions. In addition, the ability to more accurately define the use-case for a HP-BEV will underpin ongoing experimentation and mathematical modelling to quantify the associated cell ageing and degradation that may occur within HP vehicle applications

Inhibitive effect of Pt on Pd-hydride formation of Pd@Pt core-shell electrocatalysts: An in situ EXAFS and XRD study

In situ EXAFS and XRD have been used to study the electrochemical formation of hydride phases, Habs, in 0.5 M H2SO4 for a Pd/C catalyst and a series of Pd@Pt core-shell catalysts with varying Pt shell thickness, from 0.5 to 4 monolayers. Based on the XRD data a 3% lattice expansion is observed for the Pd/C core catalyst upon hydride formation at 0.0 V. In contrast, the expansion was ≤0.6% for all of the core-shell catalysts. The limited extent of the lattice expansion observed suggests that hydride formation, which may occur during periodic active surface area measurements conducting during accelerated aging tests or driven by H2 crossover in PEM fuel cells, is unlikely to contribute significantly to the degradation of Pd@Pt core-shell electrocatalysts in contrast to the effects of oxide formation