How To Improve Capacity and Cycling Stability for Next Generation Li–O₂ Batteries: Approach with a Solid Electrolyte and Elevated Redox Mediator Concentrations

Abstract

Because of their exceptionally high specific energy, aprotic lithium oxygen (Li-O2) batteries are considered as potential future energy stores. Their practical application is, however, still hindered by the high charging overvoltages and detrimental side reactions. Recently, the use of redox mediators dissolved in the electrolyte emerged as a promising tool to enable charging at moderate voltages. The presented work advances this concept and distinctly improves capacity and cycling stability of Li-O2 batteries by combining high redox mediator concentrations with a solid electrolyte (SE). The use of high redox mediator concentrations significantly increases the discharge capacity by including the oxidation and reduction of the redox mediator into charge cycling. Highly efficient cycling is achieved by protecting the lithium anode with a solid electrolyte, which completely inhibits unfavored deactivation of oxidized species at the anode. Surprisingly, the SE also suppresses detrimental side reactions at the carbon electrode to a large extent and enables stable charging completely below 4.0 V over a prolonged period. It is demonstrated that anode and cathode communicate deleteriously via the liquid electrolyte, which induces degradation reactions at the carbon electrode. The separation of cathode and anode with a SE is therefore considered as a key step toward stable Li-O2 batteries, in conjunction with a concentrated redox mediator electrolyte.

Optimized Concentration of Redox Mediator and Surface Protection of Li Metal for Maintenance of High Energy Efficiency in Li–O2 Batteries

“…[11,13,15,20,27] Generally, the RMs containing Li + source, such as LiBr and LiI, have functions of RM as well as Li salt in electrolyte. [11,13,15,20,27] Generally, the RMs containing Li + source, such as LiBr and LiI, have functions of RM as well as Li salt in electrolyte.…”

“…The cyclic voltammetry (CV) curves of tetrathiafulvalene (TTF) and 5,10-dimethylphenazine (DMPZ) revealed remarkable deactivation associated with the redox reaction, with decreasing peak intensities observed over 50 cycles (Figure S1a, Supporting Information). [11,13,15,20,27] Furthermore, LiBr contains Li ions and is highly soluble in DEGDME; thus, we selected LiBr as the concentrated RM to ensure that the RM concentration was sufficient to decompose Li 2 O 2 over a large number of cycles. [22,38] 2,2,6,6-Tetramethylpiperidinyloxy (TEMPO) and LiBr exhibit relatively stable CV curves compared with those of TTF and DMPZ (Figure S1a, Supporting Information).…”

“…[17][18][19] In addition, when direct contact between the solid catalyst and Li 2 O 2 is prevented or the surface of the catalyst is covered with Li 2 O 2 , no catalytic activation effect is expected. [20][21][22] Thus, more stable RMs are still required for practical application of Li-O 2 batteries with longer cycle life and higher efficiency.To alleviate the aforementioned problems associated with RMs in Li-O 2 batteries, Li metal, which is an essential anode component for obtaining a high energy density, should be protected through surface treatment and the employment of functional separators. RMs constitute a more efficient solution for charge-discharge reversibility, resulting in a reduction of the charge overpotential without severe electrolyte decomposition.…”

“…[10,11] Alternatively, liquid catalysts, called redox mediators (RMs), [10][11][12][13][14][15][16] have been added to the electrolyte as electron-hole transfer agents to promote reversible Li 2 O 2 formation/ decomposition. [20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] However, reported Li protection materials and Li-protective layers mainly consist of polymers, which have insufficient stiffness to suppress the growth of Li dendrites. However, it has recently been reported that RMs can be deactivated during cycling through chemical reduction at the Li metal electrode (self-discharge of the electrochemically oxidized RM) and attack by oxygen species at the cathode.…”

“…[20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] However, reported Li protection materials and Li-protective layers mainly consist of polymers, which have insufficient stiffness to suppress the growth of Li dendrites. [20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] However, reported Li protection materials and Li-protective layers mainly consist of polymers, which have insufficient stiffness to suppress the growth of Li dendrites.…”