PERSPECTIVE published: 20 February 2019 doi: 10.3389/fchem.2019.00079 Multivalent Batteries—Prospects for High Energy Density: Ca Batteries Damien Monti 1 , Alexandre Ponrouch 1 , Rafael B. Araujo 2 , Fanny Barde 3 , Patrik Johansson 2,4 and M. Rosa Palacín 1,4* 1 Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Bellaterra, Spain, 2 Department of Physics, Chalmers University of Technology, Göteborg, Sweden, 3 Battery & Fuel Cell, Advanced Material Research and Development Department, Technical Centre, Toyota Motor Europe, Zaventem, Belgium, 4 ALISTORE-ERI European Research Institute, CNRS FR 3104, Hub de l’Energie, Rue Baudelocque, Amiens, France Batteries based on Ca hold the promise to leapfrog ahead regarding increases in energy densities and are especially attractive as Ca is the 5th most abundant element in the Earth’s crust. The viability of Ca metal anodes has recently been shown by approaches that either use wide potential window electrolytes at moderately elevated temperatures or THF-based electrolytes at room temperature. This paper provides realistic estimates of the practical energy densities for Ca-based rechargeable batteries at the cell level, calculated using open source models for several concepts. The results from the Ca metal anode batteries indicate that doubled or even tripled energy density as compared to the Edited by: state-of-the-art Li-ion batteries is viable if a practical proof-of-concept can be achieved. Francesca Soavi, Keywords: calcium batteries, energy-cost model, Ca-sulfur, Ca-ion, metal anodes University of Bologna, Italy Reviewed by: Verónica de Zea Bermudez, University of Trás-os-Montes and Alto INTRODUCTION Douro, Portugal Although Li-ion batteries (LIBs) dominate the portable electronics battery market, there is a broad Claudio Gerbaldi, Politecnico di Torino, Italy diversity of alternative technologies that are successfully used in more niche applications, providing different figures of merit in terms of energy density, costs, lifetime, etc. The perspective of a novel *Correspondence: M. Rosa Palacín or old battery technology to embracing large-scale applications, such as the grid and renewable rosa.palacin@icmab.es solar and wind power, motivates the many current paths of new battery chemistries that can supersede/complement LIBs. One of the plausible solutions is to develop multivalent (Mg, Ca, Al) Specialty section: batteries which, in contrast to LIBs, would be based on the use of metal anodes (Canepa et al., 2017). This article was submitted to If successful, this concept would yield leaping breakthroughs in energy density while at the same Inorganic Chemistry, time being based on cheaper and more abundant elements. Until now, extensive efforts have been a section of the journal dedicated mainly to Mg-batteries. However, electrolyte issues—such as limited electrochemical Frontiers in Chemistry stability windows (Lipson et al., 2016)—and the lack of operational cathode materials have Received: 12 November 2018 considerably slowed down the progress in the field (Yoo et al., 2013). In stark contrast, reversible Accepted: 28 January 2019 Ca electrodeposition has only recently been unveiled, thereby opening new research avenues Published: 20 February 2019 (Ponrouch et al., 2016). Ca metal anode-based batteries would enable large gravimetric- and Citation: volumetric-specific energies, but this new technology is held back by the limited range of suitable Monti D, Ponrouch A, Araujo RB, electrolytes and cathodes despite the recently witnessed and significant technical breakthroughs Barde F, Johansson P and Palacín MR (Gummow et al., 2018; Ponrouch and Palacin, 2018). Before Ca-based batteries can enter the (2019) Multivalent Batteries—Prospects for High Energy market, electrolyte compositions are required to have electrochemical stability windows over 4 V Density: Ca Batteries. and enable Ca2+ solvation through weak coulombic interactions, improving the overall kinetics Front. Chem. 7:79. and de-solvation at the cathode surface. On the other hand, to overcome sluggish solid-state doi: 10.3389/fchem.2019.00079 diffusion, cathode materials should be developed with low migration barriers for calcium ions. Frontiers in Chemistry | www.frontiersin.org 1 February 2019 | Volume 7 | Article 79 Monti et al. Prospects for Ca Based Batteries The aim of this paper is to quantify the figures of merit attainable viability of a Ca metal anode battery (CaB) technology, including with this technology using reliable techno-economic models. selected cathodes, for which reliable data at the laboratory scale Although the Battery Performance and Cost (BatPaC) model cell level are available. (Nelson et al., 2011, 2012) was elegantly and comprehensively applied to Mg batteries (Canepa et al., 2017), no similar reports RESULTS AND DISCUSSION have tackled Ca batteries. Much simpler than BatPaC, which always considers the full battery pack, the energy-cost model Table 1 displays active material properties such as average developed by Berg et al. (2015) is employed in this study, discharge potential and specific capacity used to estimate energy considering the performance at the single electrochemical cell densities, which are subsequently normalized per volume or level (Figure 1). Hence, we avoid any possible and uncertain weight of the studied cell configuration, including current differences in electric connections and pouch packaging, and collectors, separator, and electrolyte as well as electrode materials. instead the input parameters needed are mainly operating In addition, the electrode compositions are specified for LIBs, potentials and specific capacities of the active electrode materials. SIBs, CaBs, Li-S, and also Ca-S batteries. LIBs use an aluminum Here we estimate the energy density of a set of hypothetical full current collector at the cathode, but copper at the anode, as Al Ca electrochemical cells by modifying the anode configuration, alloys with Li at potentials lower than 0.6 V vs. Li+ /Li. However, cathode specific capacities, and operating voltage. Furthermore, for SIBs Al can be used for both current collectors (as Na does the results obtained will be compared to the state-of-the-art LIBs not alloy with Al) (Ponrouch et al., 2013, 2016), and this may (Nitta et al., 2015), Na-ion batteries (SIBs) (Ponrouch et al., also be the case for CaBs. The electrode coating thicknesses 2013; Hwang et al., 2017), and Li/Ca-sulfur (Li-S/Ca-S) battery were adjusted to 100 µm for the cathode to reach a 1:1 charge technologies (Bruce et al., 2011; Hagen et al., 2015), as well as balancing—c.f. the thickness range for graphite and Ca metal to a hypothetical Ca-ion battery, with graphite as an alternative in Table 1—and for electrolyte amount and density the same as anode to Ca metal. Finally, some figures regarding cost will be for LIBs in Berg’s model were used. Two anode configurations drawn taking LiNi0.33 Mn0.33 Co0.33 O2 (lithium nickel manganese have been tested for the hypothetical Ca battery: a 50% Ca cobalt oxide, or NMC)/graphite state-of-the-art LIB technology excess with Al current collector (denoted conf. 1 in Table 1), as a reference (Shaju and Bruce, 2006), by first determining mimicking Berg’s assumptions for the Li metal anode, and a Ca configurations that can compete in terms of energy density 100% Ca excess without Al current collector (denoted conf. and then simulating different possible cathode and Ca metal 2). This excess is to compensate for the potential loss of Ca anode costs. In addition, hypothetical Ca-based systems with an consumed upon formation of a passivation layer, as is the case identical operation voltage (i.e., 3.5 V) are considered based on for the solid electrolyte interphase (SEI) creation in LIBs. In the the conditions required to have similar cost. Overall, all these second configuration, a larger excess is considered necessary to energy and cost estimates serve to survey the future market ensure proper mechanical strength of the anode in the absence FIGURE 1 | Schematic of a LIB (left) with Cu and Al current collectors and a CaB (right) with two Al current collectors. They are completed with a separator, an electrolyte, and electrodes. Each composite electrode is here composed of active material, carbon black additive, and binder. Reproduced from Palacín (2009) with permission from the Royal Society of Chemistry. Frontiers in Chemistry | www.frontiersin.org 2 February 2019 | Volume 7 | Article 79 Monti et al. Prospects for Ca Based Batteries TABLE 1 | Active materials and their properties in our cell designs. Average discharge Capacity Density Volumetric Porosity Thickness Electrode capacity potential (mAh.g−1 ) (g.cm−3 ) Expansion (%) (µm) (mAh.cm−3 ) (V vs. Li/Na/Ca) (%) incl. Carbon+Binder CATHODES LiCoO2 (LCO) 3.8 150 5.05 – 30 104 447 LiNi0.33 Mn0.33 Co0.33 O2 3.7 170 4.75 – 30 103 454 (NMC) LiFePO4 (LFP) 3.4 160 3.65 – 30 100 309 Na3 V2 (PO4 )2 F3 (NVPF) 3.8 105 4.00 – 30 100 256 Ca3 Co2 O6 3.2 160 4.52 – 30 100 435 TiS2 2.0 (Ca) 239 3.25 – 30 100 486 Li2 S 2.1 1000 1.66 80 30 100 850 CaS 1.9 1000 2.59 80 30 100 1152 Hypothetical calcium 2.0-4.5 50-300 4.5 – 30 100 135-810 cathode ANODES Graphite (Li) 0.1 (Li) 360 2.20 10 36 66-100 466 Graphite (Ca) 0.1 (Ca) 744 2.20 10 36 45 923 Hard carbon (HC) 0.3 (Na) 270 2.00 – 30 73 352 Li metal 0.0 3884 0.53 50 67 61 1383 Calcium metal (conf. 1) 0.0 1338 1.57 50 67 10-58 1400 Calcium metal (conf. 2) 0.0 1338 1.57 100 50 13-77 1050 OTHER COMPONENTS Aluminum CC 2.7 10 Copper CC 8.96 40 Separator+electrolyte 1.02 30 (sep.) 25 ELECTRODE COMPOSITIONS Standard 93 wt% Active material / 4 wt% Carbon additive / 3 wt% Binder Sulfur 60 wt% Sulfur / 30% Carbon / 10 wt% Binder of any additional current collector. Calculations were performed active at 100◦ C (Tchitchekova et al., 2018a,b), although full for hypothetical CaB cathodes with specific capacities and reversibility and cyclability remain to be demonstrated. The operating voltages ranging between 50–300 mAh.g−1 and 2.0– hypothetical Ca-ion technology enlists Ca3 Co2 O6 as the cathode 4.5 V, respectively. Such voltages should be possible by employing and a graphite anode. electrolytes composed of, for example, Ca(BF4 )2 in a (1:1) Starting with Ca3 Co2 O6 /Ca, the theoretical energy densities mixture of ethylene carbonate (EC) and propylene carbonate for the resulting CaB would top both state-of-the-art LIB and (PC) (Ponrouch et al., 2016) or Ca(BH4 )2 in tetrahydrofuran SIB technologies while being cheaper because of the lower cost of (THF) (Wang et al., 2017), with oxidation stabilities of 4.0 V Ca (3 $/ton) (Lime, 2015) compared to Na (10 $/ton) (Sodium, and 3.0 V, respectively. The difference in energy density between 2015) and Li (15,000 $/ton) ores (Lithium, 2015). Indeed, the both the two anode configurations is minor, both per volume Ca3 Co2 O6 /Ca CaB would be ∼80 Wh.L−1 and 30 Wh.kg−1 and weight, <5% even for the most extreme case—i.e., 4.5 V better than NMC or LiCoO2 (LCO)/graphite LIBs, mainly due and 300 mAh.g−1 –which results in 1,797 Wh.L−1 vs. 1,719 to the larger energy density of the Ca metal anode compared Wh.L−1 and 755 Wh.kg−1 vs. 750 Wh.kg−1 . Therefore, the to graphite (as cathode capacities, densities, and operation configuration using a current collector, as is the standard for LIBs, potentials are similar). Turning to TiS2 /Ca, this CaB arrives at was used. Figure 2 depicts operation potential vs. gravimetric 250 Wh.L−1 and 55 Wh.kg−1 worse than Ca3 Co2 O6 /Ca because and volumetric energy density for state-of-the-art LIBs as well of the lower operation potential and higher cathode density, as SIBs, and also shows the hypothetical CaB, Ca-ion, Li-S, and is also 170 Wh.L−1 below both LCO or NMC/graphite and Ca-S batteries. Specific battery technologies are depicted LIBs. However, the TiS2 /Ca CaB is better yet than both with symbols, while the hypothetical CaBs are represented by LiFePO4 (LFP)/graphite LIBs and Na3 V2 (PO4 )2 F3 (NVPF)/HC the straight lines with energy densities as a function of the (Ponrouch et al., 2013) SIBs. In the case of the hypothetical operating voltage and the specific capacity of the cathodes—the Ca-ion, the composition CaC6 was considered for the anode, capacities are specified on each line. Two specific CaBs have which has been achieved by chemical reduction of graphite in the also been considered in more detail: using TiS2 and Ca3 Co2 O6 presence of calcium (Cahen et al., 2013). The operating voltage is as cathode materials, both were shown to be electrochemically adjusted by the 0.172 V difference in standard reduction potential Frontiers in Chemistry | www.frontiersin.org 3 February 2019 | Volume 7 | Article 79 Monti et al. Prospects for Ca Based Batteries FIGURE 2 | Volumetric (A) and gravimetric (B) energy densities for LIBs (circle), SIBs (diamond), Li-S (square), and considered Ca (triangles) battery technologies. The straight lines are calculated energy densities of hypothetical CaBs as a function of operation potential and capacities (denoted on the right of each line). All calculations were made using the model developed by Berg et al. (2015). between the Li+ /Li and Ca2+ /Ca couples. As expected, the energy $.kWh−1 , were taken from Berg et al. Two approaches have been densities of the hypothetical Ca-ion concept are much lower used: (1) cathode capacities fixed at 100, 200, and 300 mAh.g−1 than the Ca metal concept: 120 Wh.L−1 and 50 Wh.kg−1 . This and the operation voltage varied to match the calculated energy originates from the somewhat lower cell potential, but foremost densities of the NMC/graphite cell (279 Wh.kg−1 . 711 Wh.L−1 ), from the lower energy density related to use of graphite. However, resulting in 4.5, 2.5, and 2.0 V, respectively; and (2) the operating the configuration would still yield higher energy densities than voltage fixed to 3.5 V—identical to the NMC/graphite cell— both LFP/graphite and NVPF/HC configurations. and cathode capacities varied at 100, 200, and 300 mAh.g−1 Furthermore, using the values for Li-S batteries, with Li metal (Figure 3). As the electrode costs vary, the colored areas in anodes calculated by Berg et al. as a starting point, we targeted Figure 3 represent the anode/cathode pairs with prices allowing an analogous Ca-S battery. The energy densities of such a Ca- CaBs to be either equal or lower in cost than NMC/graphite (110 S battery, together with the Li-S battery, are clearly higher than $.kWh−1 ). The cathode cost is the major factor given the much for any other technology considered here as both the anode and higher capacity of the Ca metal anode. By fixing the voltage to the cathode have very high capacities. However, Ca being denser 3.5 V, the energy densities in general supersede NMC/graphite than Li results in the Ca-S battery having a higher volumetric (711 Wh.L−1 and 279 Wh.kg−1 ), only the CaB using a 100 energy density but lower gravimetric energy density. Hence, there mAh.g−1 cathode yielded lower energy densities and therefore may still be advantages for large-scale applications because of limit the cathode cost to a maximum 21 $.kg−1 (Figure 3B). the lower cost of Ca-S as compared to Li-S. There are also On the other hand, the CaB configurations using 200 and 300 many applications where the volumetric energy density is just mAh.g−1 cathodes yield much higher energy densities-−1,030 as precious, or more so, than the gravimetric when a certain and 1,398 Wh.L−1 , respectively—enabling cathode costs of 50 threshold on the latter has been achieved. and 80 $.kg−1 , significantly higher than the cost of NMC (33 Overall, CaBs with only moderate operating voltages of 2.1 $.kg−1 ) (Berg et al., 2015). or 2.5 V and cathode capacities of 250 or 200 mAh.g−1 would already yield higher energy densities than the best state-of-the-art CONCLUSIONS LIB. Moreover, in specific cases such as 3.0 V/250 mAh.g−1 or 3.5 V/200 mAh.g−1 , CaBs would have volumetric energy densities Our simple estimate exercise confirms CaBs to be in principle above 1,000 Wh.L−1 , hence higher than any of the sulfur cathode- able to compete with the state-of-the-art LIBs, and this depends based technologies. For the gravimetric energy densities, 3.5 to the utmost on Ca metal anodes being viable. In contrast, Ca- V/300 mAh.g−1 or 4.0 V/250 mAh.g−1 are necessary to supersede ion batteries would fall short in figures of merit for performance Li-S and Ca-S. and would only be advantageous in terms of cost. If sulfur Second in importance, the cost-effectiveness of the cathodes were viable, coupling them to Ca metal anodes would hypothetical CaBs is calculated and compared to NMC/graphite. provide the best figures of merit for performance, similar to Li-S For the latter the cost of both electrodes, current collectors batteries. Given the relatively high price of Li metal (in addition (Cu and Al), separator, and electrolyte, yielding a total of 110 to imminent production and resource issues), cost and resource Frontiers in Chemistry | www.frontiersin.org 4 February 2019 | Volume 7 | Article 79 Monti et al. Prospects for Ca Based Batteries FIGURE 3 | Cost-effectiveness of estimated CaBs with cathode capacities of 100 (blue), 200 (orange), and 300 (green) mAh.g−1 vs. a reference NMC/graphite cell (110 $.kWh−1 ) taken as areas representing the costs of both cathode and anode (Ca metal), allowing a total cell cost equal to or lower than 110 $.kWh−1 for: (A) CaBs with energy densities similar to or higher than NMC/graphite, and (B) with an operation potential of 3.5 V, identical to NMC/graphite. The orange line represents the NMC price proposed by Berg et al. (33 $.kg−1 ). advantages for a Ca-S battery are foreseen. Overall, in terms of RA: data on electrolytes and discussion of results. FB: discussion cost, CaBs are on par with NMC/graphite, even for very high of results on global performance. PJ: design of study and cathode costs (>80 $.kg−1 ). discussion of results. MP: design of study and discussion Despite the technical breakthroughs needed, such as of results. the development of efficient Ca metal anode/electrolyte combinations operating at room temperature and cathode ACKNOWLEDGMENTS materials, CaBs are clearly a research avenue worth pursuing as, if these breakthroughs are achieved, this future emerging We authors acknowledge support from the Severo Ochoa technology would be advantageous from a techno-economic Programme for Centers of Excellence in R&D (SEV-2015-0496). point of view. DM acknowledges the EC for a H2020 MSCA-IF grant (contract number 743439). 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Sci. 6, 2361–2369. doi: 10.1039/C3EE41379A Conflict of Interest Statement: FB is an employee of Toyota Motor Europe Ponrouch, A., Frontera, C., Bardé, F., and Palacín, M. R. (2016). Towards company. a calcium-based rechargeable battery. Nat. Mater. 15, 169–172. doi: 10.1038/nmat4462 The remaining authors declare that the research was conducted in the absence of Ponrouch, A., and Palacin, M. R. (2018). On the road toward calcium- any commercial or financial relationships that could be construed as a potential based batteries. Curr. Opin. Electrochem. 9, 1–7. doi: 10.1016/j.coelec.201 conflict of interest. 8.02.001 Shaju, K. M., and Bruce, P. G. (2006). Macroporous Li(Ni1/3Co1/3Mn1/3)O2: a Copyright © 2019 Monti, Ponrouch, Araujo, Barde, Johansson and Palacín. This high-power and high-energy cathode for rechargeable lithium batteries. 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