Sensitivity Analysis and Joint Estimation of Parameters and States for All-Solid-State Batteries

Design of fast and efficient algorithms for adaptive observers, i.e., joint estimation of parameters and internal states of a dynamical system is vital for various applications. In this paper, a recently proposed initial excitation-based adaptive observer is modified to use a matrix regressor, instead of a vector regressor, for fast on-line estimation. A novel concept of strong initial excitation condition (S-IE) is formulated, leading to a switched estimation scheme, which is shown to be milder than already existing excitation conditions like persistence of excitation (PE), strong persistent excitation (S-PE) and initial excitation (IE). The proposed algorithm enables exponential convergence by a user-assignable convergence rate. Strategic introduction of estimator for the unknown initial conditions of the state variables in conjunction with the parameter estimator ensures uniformly global exponential stability (UGES) of the estimation error dynamics. The proposed adaptive observer scheme is validated using a linear plant dynamics along with an electro-chemical model of Lithium-ion (Li-ion) battery for the estimation of state of charge (SoC) and state of health (SoH).

Accurate lithium-ion battery charge state estimation is crucial for battery management systems. Modeling of dual polarization—electrical equivalent circuit based on ternary lithium batteries as a research object, a variable forgetting factor recursive least square method is proposed for parameter identification given the insufficient tracking ability of the traditional recursive least squares method for abrupt and time-varying signals in a non- stationary environment. A backward smoothing square root cubature Kalman filtering algorithm is applied to enhance the accuracy and convergence speed of SOC estimation. The algorithm uses the square root update to ensure the numerical stability of the filtering and uses the idea of backward smoothing-forward filtering to improve the filtering accuracy on the basis of the first forward filtering. Finally, variable forgetting factor recursive least square is combined with backward smoothing square root cubature Kalman filtering to achieve the joint estimation of model parameters and state of charge, and the feasibility of the battery state of charge estimation is verified in different working conditions. The simulation results show that the variable forgetting factor recursive least square-backward smoothing square root cubature Kalman filter algorithm improves the study’s filtering accuracy and convergence speed of lithium-ion batteries.

There is a growing interest in the sustainability of the aviation industry sector over the past years due to the environmental issues associated with traditional aviation engines. Electric and hybrid aircrafts are considered promising technologies for reducing fuel consumption and enhancing system efficiency [1]. However, electrical energy storage systems require a higher capacity-to-weight ratio than today’s Li-ion batteries to fulfil the high demands in this area. Safety restrictions imposed by liquid electrolytes motivate the development of next-generation chemistries, such as oxide-based all-solid-state batteries (ASSB) for aviation, which have non-flammable electrolytes [2]. This option is investigated in the context of the IMOTHEP European project that aims at identifying promising hybrid aircraft configurations and studying the associated technology. However, the major drawbacks of oxide-based solid electrolytes are weak contact between electrode and electrolyte interface, low mechanical flexibility, and high density, which limit their use for high gravimetric energy density applications. To mitigate the aforementioned concerns, the solid polymer composite electrolytes approach could be applied, where oxides are mixed with polymer electrolytes [3].Designing an optimum cell without ion transport limitations using experimental investigations is time- as well as resource-intensive due to the large number of iterations in production and evaluation required to achieve a well-performing design. Physics-based modelling is able to create a platform that can directly assess the impact of cell structure on battery performance and provide knowledge concerning limiting processes within the cell. Therefore, we here present the first study that combines a pseudo-two-dimensional model for the model-assisted evaluation of Li-ASSB with various hybrid electrolytes and single-ion conductor electrolytes with an evolutionary algorithm to identify optimum cell designs to reach a higher gravimetric energy density (see Fig. 1-a). To this end, we first compared the performance of several hybrid electrolytes with their experimental properties, to identify which electrolyte performs well with present technology and which has the potential to become an attractive alternative in the future.Our findings reveal that based on available ASSB technology, single ion-conducting electrolytes cannot achieve a higher gravimetric energy density than hybrid electrolytes at low current rates due to their high density, as shown in Fig. 1-b. ASSB based on 12.7 vol% of garnet Li6.4La3Zr1.4Ta0.6O12 (LLZTO) is the best option based on present manufacturing constraints. Furthermore, our study revealed that hybrid electrolytes based on 10 wt% of Li1.3Al0.3Ti1.7(PO4)3 (LATP) could be promising for future aircraft if researchers succeed to decrease its electrolyte thickness and chemical stability in contact with lithium metal anode. Further, sensitivity analyses enabled us to identify that the cathode thickness and volume fraction of cathode materials are critical parameters for the cell design of ASSB. Therefore, we applied a global optimisation to enhance gravimetric energy density by changing these two electrode design parameters. After optimisation, gravimetric and volumetric energy densities of 618 Wh kg-1 and 1251 Wh L-1 for 0.1C discharge are achieved, respectively, indicating that the cell with the optimal electrode design could meet the mission demand in the aviation industry with a gravimetric energy density of 500 Wh kg-1 and volumetric energy density of 1000 Wh L-1.In conclusion, the findings of this study show that our physics-based modelling in conjunction with an optimisation algorithm predicts the optimal composition of ASSB for a given constraint and thus supports the time- and cost-effective development of batteries that fulfil mission requirements, e.g. in the aviation sector.This work is conducted in the frame of the project IMOTHEP (Investigation and Maturation of Technologies for Hybrid Electric Propulsion), which has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 875006 IMOTHEP.

Nature of LiFePO4 aging process: Roles of impurity phases

Silicon is a promising material for high-energy anode materials for the next generation of lithium-ion batteries. The gain in specific capacity depends highly on the quality of the Si dispersion and on the size and shape of the nano-silicon. The aim of this study is to investigate the impact of the size/shape of Si on the electrochemical performance of conventional Li-ion batteries. The scalable synthesis processes of both nanoparticles and nanowires in the 10–100 nm size range are discussed. In cycling lithium batteries, the initial specific capacity is significantly higher for nanoparticles than for nanowires. We demonstrate a linear correlation of the first Coulombic efficiency with the specific area of the Si materials. In long-term cycling tests, the electrochemical performance of the nanoparticles fades faster due to an increased internal resistance, whereas the smallest nanowires show an impressive cycling stability. Finally, the reversibility of the electrochemical processes is found to be highly dependent on the size/shape of the Si particles and its impact on lithiation depth, formation of crystalline Li15Si4 in cycling, and Li transport pathways.

Due to their high capacity, Silicon (Si) anodes are very promising for the next generation of lithium-ion batteries. However, Si materials suffer from a high volume expansion, which cause cracking and accumulated growth of the solid electrolyte interphase (SEI), resulting in low coulombic efficiencies and lifetime of the Si anodes. Applying additives in the electrolyte is a facile and straightforward approach to improve the composition of the SEI and reduce cracking. For Si-containing anodes, fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are considered the most effective electrolyte additives. The salt lithium difluorophosphate (LiDFP) is known to improve the interphases of cathode materials and graphite when applied as additive, but has been poorly investigated for Si anodes. In our study, we combine VC, FEC, and a variation of LiDFP concentrations to investigate the effect on Si-dominant anodes in detail. We conducted detailed electrochemical testing in half coin cells and Si/NCM pouch cells, including long-term cycling, rate capability tests, potential monitoring of the anode and electrochemical impedance spectroscopy. For the latter two, we applied a novel perforated reference electrode concept, which allows for broad and precise measurement in pouch cells, while avoiding a blocking effect of the lithium ions. Furthermore, post-mortem analysis was conducted to gain detailed insights into the SEI formation and microstructure influenced by the different additive concentrations.

Atomic layer deposition of amorphous iron phosphates on carbon nanotubes as cathode materials for lithium-ion batteries

Li-rich layered oxides (LLOs) are considered as the cathode materials for the next generation of lithium-ion batteries because of their high specific capacity. However, due to the structural rearrangement in the charging and discharging process, the initial Coulombic efficiency (ICE) is low and accompanied by serious voltage attenuation and bad cycle performance. In this work, the Li-gradient layer on the surface of the rod-like LLOs is constructed by the treatment of (NH4)2SiF6 under high temperature and subsequent washing. XRD, HRTEM, and Raman spectra verify the existence of the spinel phase of Li1–xMn2O4 on the surface of LLOs by the modification. The results of electrochemical measurements show that the modified LLOs exhibit a higher ICE of 87% with an excellent cycling retention of 90.4% (after 100 cycles at 1 C). These improvements on the electrochemical performance are caused by the high Li+ diffusion and improved the redox reversibility of oxygen ions on the LLOs with the unique structure of the Li-gradient and spinel coating layer on the surface.

The spinel Li4Mn5O12 has been considered as a prospective 3 V cathode material for the next generation of lithium-ion batteries (LIBs) due to its high energy density and excellent cycling stability. However, the low operating voltage (∼3 V) makes Li4Mn5O12 impractical for high-energy high-power LIBs. To address this issue, Ni and Fe dual doped Li4Mn5-x-yNixFeyO12 has been prepared via a facile sol-gel method combined with post-heat-treatment. The effects of dual-cations doping on the crystal structure, morphology and electrochemical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and galvanostatic charge/discharge analysis. As a result, Li4Mn4Ni0.5Fe0.5O12 exhibits the highest reversible specific capacity of 133 mAh/g at a specific current density of 25 mA/g after 100 cycles and exhibits a significantly improved high voltage performance with corresponding capacity of ∼80 mAh/g at an average voltage of 4.7 V vs. Li/Li+ and ∼122 mAh/g at above 4.0 V. These results indicate the dual doping of Ni and Fe can effectively improve both the operating voltage and reversible specific capacity of Li4Mn5O12 with excellent cycling stability, demonstrating a promising high-voltage cathode material for high-energy high-power LIBs.

Thermodynamic Analysis of Lithium-Ion Battery Storage System

Lithium-rich manganese-based cathodes (LR) are valuable cathode materials for the next generation of lithium-ion batteries (LIBs) with high-energy density. However, the fast voltage/capacity decay on cycling is the major obstacle for the practical application induced by the less-than-ideal anionic redox reactions and structure distortion. Herein, in order to tackle these challenges, a perovskite-like La2Li0.5Co0.5O4 (LLCO) material is selected as protective surface to stabilize the Li1.2Mn0.54Ni0.13Co0.13O2 (LR) substrate through wet chemical coating method. Versatile structure/phase characterizations and electrochemical tests exhibit that the LLCO can not only minish the oxygen evolution and enhance the structure stability, but also restrain the electrolyte corrosion and increase the mechanical strength of cathode materials. Moreover, the coated LLCO with high electronic/ionic conductivity dramatically accelerates the energy storage kinetic, thereby displaying the improved rate performance. Specifically, the optimized LR@LLCO sample (1LLCO) exhibits a high capacity of 250.6 mAh·g−1 after 100 cycles at 0.1 C and excellent capacity retention of 82.6% after 200 cycles at 2 C. This work provides a new idea for the modification of LR cathodes toward commercial high-performance LIBs.

Li-rich Mn-based layered oxides (LMLOs) are expected to be the most promising high-capacity cathodes for the next generation of lithium-ion batteries (LIBs). However, the poor cycling stability and kinetics performance of polycrystalline LMLOs restrict their practical applications due to the anisotropic lattice stress and crack propagation during cycling. Herein, B-doped micron-sized single-crystal Co-free LMLOs were obtained by molten-salt (LiNO3 and H3BO3)-assisted sintering. The results reveal that the low-melting-point molten salt can serve as liquid-phase media to improve the efficiency of atomic mass transfer and crystal nucleation and growth. The modified single-crystal LMLO cathodes can resist the accumulation of anisotropic stress and strain during the cycling and reduce interface side reactions, thus achieving excellent high-voltage stability and kinetics performance. The reversible specific capacity of the single crystals is 210.8 mAh g-1 at 1C with a voltage decay rate of 1.95 mV/cycle and up to 161.1 mAh g-1 at 10C with a capacity retention of 81.06% after 200 cycles.

Li-rich manganese-based cathode materials (LRMs) are considered one of the most promising cathode materials for the next generation of lithium-ion batteries (LIBs) because of their high energy density. However, there are problems such as a capacity decay, poor rate performance, and continuous voltage drop, which seriously limit their large-scale commercial applications. In this work, Li1.2Mn0.54Co0.13Ni0.13O2 coated with Li2MoO4 with a unique spinel structure was prepared with the wet chemistry method and the subsequent calcination process. The Li2MoO4 coating layer with a spinel structure could provide a 3D Li+ transport channel, which is beneficial for improving rate performance, while protecting LRMs from electrolyte corrosion, suppressing interface side reactions, and improving cycling stability. The capacity retention rate of LRMs coated with 3 wt% Li2MoO4 increased from 69.25% to 81.85% after 100 cycles at 1 C, and the voltage attenuation decreased from 7.06 to 4.98 mV per cycle. The lower Rct also exhibited an improved rate performance. The results indicate that the Li2MoO4 coating effectively improves the cyclic stability and electrochemical performance of LRMs.

Lithium silicide (LixSi) is the lithiated form of silicon, one of the most promising anode materials for the next generation of lithium-ion batteries (LIBs). In contrast to silicon, LixSi has not been well studied. Herein we report a facile high-energy ball-milling-based synthesis of four phase-pure LixSi (x = 4.4, 3.75, 3.25, and 2.33), using hexane as the lubricant. Surprisingly, the obtained Li3.75Si phase shows significant downward shifts in all X-ray diffraction peak positions, compared with the standard. Our interpretation is that the high-energy ball-mill-synthesized Li3.75Si presents smaller internal pressures and larger lattice constants. The chemical-stability study reveals that only surface reactions occur after Li4.4Si and Li3.75Si are immersed in several battery-assembly-related chemicals. The thermal-stability study shows that Li4.4Si is stable up to 350 °C and Li3.75Si is stable up to 200 °C. This remarkable thermal stability of Li3.75Si is in stark contrast to the long-observed metastability for electrochemically synthesized Li3.75Si. The carbon encapsulation of Li4.4Si has also been studied for its potential applications in LIBs.

Layered Li-rich Mn-based oxides are believed to be a good candidate for cathode material in the next generation of lithium-ion batteries. However, they have some disadvantages, such as low initial coulombic efficiency, low rate capacity, and deficient cyclability. To overcome these shortcomings, various approaches, such as elemental doping, have been adopted. In this study, Al-doped Li1.21Mn0.54Ni0.125Co0.125O2 were successfully synthesized using the sol–gel method. Samples were characterized by thermal analysis (TGA/DTA), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), surface-area analysis, field emission scanning electron microscopy and transmission electron microscopy (TEM). Measurements of galvanostatic charge discharge and electrochemical impedance spectroscopy were also performed to evaluate the electrochemical performance of the prepared samples. The XRD patterns showed that all the samples with the structure of 0.55Li2MnO3.0.45LiNi0.33Mn0.33Co0.33O2 had a composite material with two individual layered structures that are integrated with each other. By doping Al, the lattice parameters of the samples changed. The first discharge capacity of the Al-doped specimens was lower than that of the pristine sample. In cycling performance results, it is clear that cyclic behavior and capacity stability rate in doped samples have improved compared to the undoped sample, and in the meantime, the sample with 0.05 aluminum doping has shown the best performance. Optimal performance of the doped specimens can be related to lower load transfer resistance and better structural stability.