LIBs have been commercially introduced by Sony since the early 1990s. To date, LIBs have been developed as one of the most important battery technologies dominating the market [22]. Generally, LIB technology is based on lithium-intercalation compounds. As shown in the schematic of LIBs (Fig. 1 [23]), lithium ions migrate through the electrolyte that is located between anode and cathode. During the discharge process, lithium ions are readily released from the anode and diffused into the delithiated cathode, which are related to the oxidation and reduction of two electrodes, respectively [5, 24].
Fig. 1Reproduced with permission [23]. Copyright 2012, The Royal Society of Chemistry
A schematic illustration of the working principle of LIBs based on the LixC6/Li1−xCoO2 cathode. During the discharging process, lithium ions are released from a lithiated graphite (LixC6) anode to a delithiated Li1−xCoO2 cathode. During the charging process, the reaction is reversed.
Full size image
Typically, in LIBs, anodes are graphite-based materials because of the low cost and wide availability of carbon. Moreover, graphite is common in commercial LIBs because of its stability to accommodate the lithium insertion. The low thermal expansion of LIBs contributes to their stability to maintain their discharge/charge capacity even after long discharge/charge cycles. However, the capacity of graphite to accommodate the lithium insertion (372 mAh/g) is relatively low, and LIBs will attract more attention if this property is improved [25]. Fortunately, in recent years, considerable efforts have been exerted to optimize anode materials based on graphite, and several new anode materials, including silicon, alloy, and metal oxides, are developed [26,27,28,29]. The capacity and lifetime of commercial LIBs have been effectively improved through the development of novel anode materials (e.g., silicon/carbon composite) or new nickel-rich cathode materials [30].
The name of current commercial LIBs originated from the lithium-ion donator in the cathode, which is the major determinant of battery performance. Generally, cathodes consist of a complex lithiated compound material, particularly several lithium metal oxide materials, such as LiCoO2, LiMn2O4, and LiFePO4 [31,32,33]. With different cathodes, battery performance significantly differs. However, compared with metallic lithium, all of the aforementioned compounds show high impedance because of their low diffusion coefficients and ionic conductivities, which will result in low EE and lifetime. This limitation can be overcome by fabricating the cathode from finely powdered lithium compound materials and blending with conductive materials (e.g., carbon) by mixing with a binder (e.g., polyvinylidene fluoride) and a solvent (e.g., N-methyl-2-pyrrolidone) [34]. The cathode on Al foil is formed into plate or spiral shape.
The electrolytes in LIBs are mainly divided into two categories, namely liquid electrolytes and semisolid/solid-state electrolytes. Usually, liquid electrolytes consist of lithium salts [e.g., LiBF4, LiPF6, LiN(CF3SO2)2, and LiBOB], which are dissolved in organic carbonates (e.g., ethylene carbonate, propylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and their mixtures) [35]. Typically, the semisolid/solid-state electrolytes are composed of lithium salts as the conducting salts and high-molecular-weight polymer matrices (e.g., polyvinylidene fluoride, poly(ethylene oxide), and polyvinylidene fluoride–hexafluoropropylene) [36, 37].
As aforementioned, in the electrical energy transformation process, grid-level energy storage systems convert electricity from a grid-scale power network into a storable form and convert it back into electrical energy once needed. Energy storage systems in the power grid need to meet the balance of electricity demand and supply in the grid. Therefore, to comply with the applications to grid-level energy storage systems, gravimetric energy density needs to be considered [14]. High EE and long cycle life are also needed [38]. In addition, a low cost and safe battery module is critical for building a high-efficiency battery system in large-scale energy storage.
Generally, the types of commercial LIBs currently used are coin, cylindrical, prismatic, and pouch (Fig. 2 [39]). In most cases, cylindrical cells follow a standard model size, i.e., 18650 cells, such as those used in Tesla cars [40]. Typically, during assembly at high tension, 18650 cell batteries deliver a 20% higher volumetric energy density of up to 600–650 Wh/L than prismatic and pouch cells [41]. Although cylindrical cells show higher energy densities, prismatic and pouch cells are more widely used because of the reduced module-level dead volume and higher design freedom. In addition, compared with cylindrical cells, prismatic-type and pouch-type batteries can be easily customized for specific products.
Fig. 2Reproduced with permission [39]. Copyright 2019, Wiley
Schematic of a coin-type, b cylindrical-type, c prismatic-type, d pouch-type batteries.
Full size image
Presently, commercially available LIBs are based on graphite anode and lithium metal oxide cathode materials (e.g., LiCoO2, LiFePO4, and LiMn2O4), which exhibit theoretical capacities of 372 mAh/g and less than 200 mAh/g, respectively [21]. However, state-of-the-art LIBs showing an energy density of 75–200 Wh/kg cannot provide sufficient energy for use in grid-level energy storage. To further improve the specific energy of LIBs, many alternatives to graphite with higher specific capacity are under exploration. For example, silicon shows high potential as a promising anode material that deliver a high theoretical capacity of 4200 mAh/g and attractive operating voltage (approximately 0.3 V vs. Li/Li+) [21]. In previous work, on the basis of an anode of 50% replacement of graphite with commercial SiOx and a cathode of LiNi0.8Co0.1Mn0.1O2 electrodes with high capacity, the energy density of a pouch-type battery configuration is predicted to increase by 7.6% [40]. Moreover, the cycle life of LIB is significantly attractive for use in grid-level energy storage as high as 10,000 cycles.
In addition to the cycle life described previously, the calendar life performance of LIBs needs to be analyzed when they are applied to grid-level energy storage systems where the maintenance or replacement of batteries demand a high cost. Calendar life refers to both the storage duration and the periodical discharge test, which should also be considered as it causes the capacity loss of the battery by self-discharging [42]. In 2017, Kubiak et al. [43] investigated the effects of self-discharging after a 3-year standby field deployment of a 250 kW/500 kWh LIB integrated with the grid and solar farm under the harsh climate conditions of Qatar. After testing, the residual capacity of LIB stack was evaluated to 93% of its initial available capacity, indicating its potential. However, it should be noted that several battery units have been damaged by self-discharging. Capacity decrease and power fading originate from the electrodes and electrolytes and the interfacial matching between them. For the electrodes, the dominant mechanism is as follows [44]: (1) contact loss of active material particles and decomposition of electrode materials (e.g., binder and additives) due to volume changes during cycling; (2) continuous solid–electrolyte interface (SEI) formation and growth leads to impedance increase at the electrodes; and (3) reactions of lithium with electrodes leading to the loss of mobile lithium. With respect to the electrolytes, electrolyte decomposition is the major cause for capacity loss, resulting in metal dissolution, migration of soluble species, precipitation of new phases, gas evolution, and surface layer formation. Moreover, the storage temperature has a significant effect on the calendar life of LIBs. For example, Asakura et al. [45] investigated the capacity retention of LP10-type LIBs under float charging conditions. They observed a capacity loss of 30% in 12 months at 45 °C even under mild conditions. Therefore, ongoing efforts are desired for exploring the self-discharging mechanism and designing advanced electrodes and electrolytes to promote the practical use of LIBs in power grids.
As mentioned previously, several unwanted/parasitic reactions include SEI growth, electrolyte decomposition, and electrode dissolution during cycling of LIBs leading to capacity loss. Coulombic efficiency (CE), which is expressed as the ratio of the discharged capacity to the capacity necessary to charge the material/system, can be used to measure the reversibility of the redox reactions [46]. Typically, graphite-based anodes exhibit high initial CEs, i.e., in the range of 95–99%. Analogous to CE, EE, which represents the ratio of the discharge energy to the charge energy, is also a key performance indicator of LIBs because electrical energy can transform into another form of energy, such as thermal energy. Meister et al. [46] analyzed the CE and EE of different anode materials. The comparison of the intercalation/insertion materials graphite and soft carbon shows nearly comparable values for CE. After the first formation cycles, the CE increases to approximately 100%. With respect to EE, graphite and soft carbon show the values of 93.8% and 93.0%, respectively. In addition, the lithium-rich cathode materials exhibit high CE and EE of approximately 99% and more than 90%, respectively, surpassing other competitive battery systems (e.g., lead–acid and nickel metal hydride batteries). In practical use, low EE will be reflected by high extra energy costs, particularly for grid-level energy storage. Therefore, LIBs with high efficiency, long cycle life, low self-discharge, and high specific energy are promising for grid power supply.
Although LIBs dominate the market, they also encounter serious challenges in realizing their wide-scale use. The major limitation is their high cost, which can be attributed to the scarcity of lithium metal resources, specific packaging, and internal protection circuits preventing overcharge [1]. Measuring the lifetime cost (in $/kWh) to understand the system economics is critical. To calculate the lifetime cost, the sum of the battery, installation, and transportation costs can be multiplied by the number of times that a new system is required over the project period, including the original install. Albright et al. [47] analyzed the lifetime cost of LIBs with the battery cost of approximately $600/kWh, installation cost of approximately $3.6/kWh, and transportation cost of approximately $5/kWh. Many efforts have been exerted to reduce the manufacturing cost of LIBs to capture future energy markets. In the USA, a project to design and construct LIBs as an energy storage system for providing power in grid-connected micro turbine applications has been sponsored by the Department of Energy and SAFT and SatCon Power Systems [1]. Moreover, a previous study reported that a demand of 100 GWh is expected with a cost level of approximately 100 €/kWh for stationary storage by 2025 [48].
In addition, LIBs are composed of highly active materials that are in contact with a flammable organic electrolyte. When they are subjected to conditions that are improperly designed, LIBs will fail prematurely. In particular, the reactions of charged positive and negative electrodes with electrolytes at elevated temperature easily result in incidents and safety issues. A previous work [49] showed that all of these materials begin reacting with the electrolyte at approximately 80 °C at a low rate, which explains the phenomenon that LIBs begin to lose capacity when cycled at temperatures higher than 60 °C. As the temperature increases, the reaction rate increases considerably. Moreover, any irregular use, such as disposing in unsafe environment with fire, excessive charging or discharging (e.g., overcharging and external short circuiting), and crushing, will result in spontaneous heat-evolving reactions, which can trigger fire or even an explosion [50]. Therefore, LIBs must pass a number of safety tests before they can be certified for use in grid-scale energy storage. The safety test must include electrical (e.g., short circuit and abnormal discharging and charging), mechanical, and environmental tests (e.g., temperature and altitude), which help determine the performance limits and ensure the working safety of LIBs.
For more information Lithium-Ion Solar Batteries, AC Coupled Inverter, please get in touch with us!