Alginate Biopolymer Binders As Manganese Ions Scavengers Improving Cycle Lives of LiMn2O4 Cathodes at High Temperature

Due to a combination of multiple advantages including low cost, abundance of raw material, high rate capability, and a plethora of work historically (~30 yrs), the manganese spinel cathode is still one of the most promising cathode materials for emerging large-scale lithium battery applications such as electrical vehicles and smart grid systems. Despite the numerous advantages addressed above, LiMn₂O₄ suffers from a limited cycle life originating from severe Mn dissolution. Although a number of approaches have been developed to resolve the issues originating from the Mn dissolution, most of them result in a sacrifice of other electrochemical properties (i.e., capacity) or to include additional synthetic steps.

Diverse approaches have been attempted to solve the challenges associated with the Mn²⁺ dissolution. They include for instance: 1) partial substitution of Mn with different metal ions such as Al³⁺, Cu²⁺, and Ti⁴⁺, 2) control of Mn valence state by Li-rich phases, 3) surface coating with metal oxides, 4) partial substitution of O with F, and 5) functional electrolyte additives. Despite improved cycling performance, many of these approaches also raised additional drawbacks, such as sacrifice of specific capacity or involve more complex steps in battery electrode or cell preparation that inevitably lead to a cost increase.

In the present study, we found a novel approach of using alginate extracted from brown seaweed as a binder in the electrode. Alginates, which are natural polysaccharides isolated from Phaeophycease, a brown algae, are most abundant bio-polymers in nature. Di- or multi-valent metal ions can trigger the formation of alginate hydrogels or precipitation of polymers by ionic interaction between metal ions and the carboxylic groups in alginates. Such a binding process has been known as the so- called “egg-box” model. The “egg-box” allows alginates to function as promising adsorbents binding toxic heavy metals such as mercury, lead, nickel, zinc, cadmium, and manganese, from industrial wastewater. Along with this line, alginate captures Mn²⁺ ions through the well-known “egg box” mechanism and indeed results in improvement of the cycling performance dramatically. A series of experiments confirmed exceptional Mn chelation capabilities of alginate. Moreover, alginate is ~10 times cheaper than conventional PVDF binder. The current approach requires simple replacement of binder from the existing manufacturing processes and should thus be readily applicable. The green and low-cost characteristics of alginates are also outstanding advantages.

Figure 1 (a) Brown algae forest in the Pacific Ocean. (b) The chemical structure of the alginate consisting of the mannuronic (M, left) and guluronic (G, right) acid units, where the alginate gel is formed by constituting an egg-box structure via chelation of Mn²⁺ ions. The blue numbers indicate the location of each carbon atom. Schematic representations of LIBs employing (c) the alginate binder (yellow) and (d) the PVDF binder (green), illustrating the Mn scavenging effect of the alginate during cell operations.