Current status and future perspectives of lithium metal batteries

TLDR: In this paper, the main advances in this field since the first attempts in the mid 1970s are reviewed, including specific applications in all-solid-state (inorganic and polymeric), Li-Sulfur (Li-S) and Lithium-O2 (air) batteries.

About: This article is published in Journal of Power Sources.The article was published on 2020-12-31 and is currently open access. It has received 85 citations till now. The article focuses on the topics: Battery (electricity).

Figures

Fig. 7. Overview of the main stabilization methods for lithium metal anodes in liquid electrolyte being adapted for Li–S batteries. a)”in-situ” SEI with carbonates for confined sulfur species in the cathode pores, additives for in-vivo SEI formation and sparingly polysulfide solvating electrolytes as approach to intrinsically hinder anode corrosion by polysulfides. b) “ex-situ” artificial SEI by gassing, sputtering of inorganic layers, and application of polymers. c) host engineering by conductive and non-conductive frameworks, spacer concepts, and alloys.

Table 1 Current status of Li-metal batteries compared to the performance targets of the EU Integrated Strategic Energy Technology Plan (SET-Plan) Action 7 for 2030 [27]. Considering the relatively low TRL of some cell chemistries, cost and manufacturing targets are omitted. For the same reason calendar life is omitted from the performance targets and only values at the cell level are compared.

Fig. 1. Comparison of roadmaps and targets of different R&D programs worldwide. Evolution of battery chemistry is also depicted. Plot modified from the Battery 2030+ Roadmap [28]. Some of the data originally provided by Hong Li et al. [26].

Fig. 6. (a) Schematic of the lithium-metal battery prototype developed in the 1990s, comprising a Li anode, a PEO-LiClO4/LiTFSI electrolyte, and a VOx cathode [236]. (b) Schematic of assembly of an LMP® battery pack (developed and commercialized by the Bolloré Group) from the cell level. Reproduced with permission from Refs. [221]. © Springer Nature Switzerland AG 2019. (c) Bluecar and (d) Bluebus (using the LMP® technology) commercialized by the Bolloré Group. Reproduced with permission from Ref. [237]. © 2020 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim.

Fig. 2. Schematic drawing showing the main stabilization routes for lithium metal in liquid and all-solid-state battery cells. For liquid cells, lithium metal can be stabilized with a host structure, “in-situ” SEI or “ex-situ” artificial SEI. All-solid-state cells can either use an inorganic or polymeric solid electrolyte to stabilize the lithium metal anode.

Fig. 8. a, True capacity of a Li–O2 cathode as a function of capacity per mass of carbon for three cases of initial porosity (given by percentages above the curves), which is filled up to 80%. The black squares and red circles at 1000 mAh gcarbon− 1 illustrate that respective true capacities vary strongly with electrode architecture. Value for the intercalation material LiFePO4 is given by the dashed line for comparison. The insert shows the space filling of spherical Li2O2 particles inside the porous electrode and the displaced electrolyte volume at 25,000 mAh gcarbon− 1 for 92% initial porosity (indicated by the green circle). Adapted with permission from Ref. [339], NPG. b, Typical moles of O2 and Li2O2 involved upon discharge and charge. Adapted with permission from Ref. [341], NPG. c, Thermodynamics of alkali peroxides and superoxides. Standard potentials of the O2/MO2 and O2/M2O2 redox couples on the M/M+ scales with M = Li, Na, K. The scales are brought to a common scale based on their M/M+ standard potentials. The dashed horizontal line indicates the O2/KO2 couple. The O2/LiO2 potential is adopted from Ref. [342]. With permission from Ref. [343], Royal Society of Chemistry. d, Overview of the discharge and charge process with dominant disproportionation steps for the 2nd e– transfer and concurrent 3O2/1O2 release (centre). From Ref. [343], Royal Society of Chemistry. Steps are more detailed for discharge/charge on top/bottom. e, Parameters determining product morphology and degree of pore filling. f, Processes upon mediated discharge and charge and open questions regarding 1O2 formation. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Frequently asked questions

Q1. What have the authors contributed in "Current status and future perspectives of lithium metal batteries" ?

• The historical development of lithium metal batteries is briefly introduced. 

Q2. What is the key to achieving the performance target of the SET plan?

Generation 5 batteries relying on conversion cathodes may be the key to achieve and, in theory, well exceed the performance target of the SET plan. 

Q3. What is the effect of coating the lithium metal anode with an alloy?

it should be noted that alloys increase the anode potential and lower the overall voltage window, so coating only the lithium metal anode’s surface with the alloy impacts less the overall energy density than using a completely alloyed anode. 

Q4. What is the way to prolong the cycling stability of lithium?

It is important that the 1-pentylamine concentration is sufficiently high (1 M) to produce a stable, homogenous surface prolonging cycling stability. 

Q5. What is the effect of the reduction of the AlI3 salt on the surface of the lithium?

The initial reduction of the AlI3 salt leads to the formation of a stable LiI layer on top of the lithium metal surface, reducing the activation barrier for Li+ transport across the electrode/electrolyte interphase. 

Q6. What is the way to adjust the solubility of polysulfide?

The combination of symmetric and “non-symmetric” ethers with varying alkyl chain lengths may also be useful to adjust the polysulfide solubility [286]. 

Q7. What are the main issues that hindered the development of a lithium metal ASSB?

Several other chemistries, including poly(methyl methacrylate) (PMMA) [229] as well as cross-linked polymers and copolymers [230], have been proposed for allowing a lithium metal ASSB; however, various issues, such as low conductivity, modest chemical stability, and scarce mechanical strength, hindered their diffusion and practical application. 

Q8. What is the description of the physicochemical properties of dry polymer membranes?

Dry polymer membranes with suitable physicochemical characteristics may be indeed processed into thin separators acting as host for lithium ions, which can move under an electric field [187]. 

Q9. What is the impact of filling the pores with inactive electrolyte on the battery?

filling the pores with inactive electrolyte may increase mass and practical implementation in the prototype cells need to be considered. 

Q10. What is the impact of the anode on the energy density of the cells?

On the other hand, lithium excess as well as protective coatings, frameworks, new electrolytes etc. may impact the energy density of the cells. 

Q11. What is the way to improve the wettability of lithium metal anodes?

While components such as the solid electrolyte and the cathode can benefit from well established processing routes such as wet chemical processing or high-viscosity processing (solvent free), research on the anode is still undergoing to find cheaper routes for the large-scale production of electrochemical and mechanically stable lithium metal anodes for ASSBs [117]. 

Q12. What is the way to achieve improved conductivity and mechanical stability?

Improved conductivity and mechanical stability can be actually achieved by employing ceramic fillers of various nature [202], e.g., Lewis acid or bases, such as Al2O3 [197], ZrO2 [203], TiO2 [204], and SiO2 [205], as well as functionalized fillers [206] and nano-sized oxides, to obtain nanocomposite polymer electrolytes (NCPEs, Fig. 5a.) [207]. 

Q13. How was the surface Li2O2 seen in low DN solvents?

Disproportionation in high DN solvents was shown via SEM, where after charging large Li2O2 particles, nanocrystalline, lamellar Li2O2 was seen. 

Q14. What are the main obstacles that have to be overcome in the short-to-medium term?

pre-commercial cells and emerging technologies lately on the market represent useful examples to identify the main obstacles that have to be overcome in the short-to-medium term for matching the economic and environmental targets of the European SET Plan. 

Q15. What is the importance of evaluating the proposed strategies in regard to a multi-layered cell?

It is vital, however, to evaluate the proposed strategies in regard of a multi-layered cell as a system comprising both active and inactive components [258–261]. 

Q16. What are the main challenges that need to be addressed to enable Li metal anodes?

As recently discussed by Cui et al. [10], among all challenges identified in the past decades, two main issues need to be addressed to enable Li metal anodes: (i) the formation/disappearance of the full volume, and (ii) the high chemical reactivity.