If you pay attention to only one battery chemistry paper per month, this would be my pick for the month of July. Harvard has been producing some very interesting work on flow battery chemistry using organic molecules over the past few years.

January 2014: A metal-free organic–inorganic aqueous flow battery

https://www.nature.com/articles/nature12909

They were using a quinone plus an acidic Br2/Br- redox couple. It was interesting because it relied entirely on abundant elements, but acidic Br2/Br- is very corrosive. And somewhat hazardous. It seems like this chemistry would demand rather expensive supporting materials/systems.

September 2015: Alkaline quinone flow battery

http://science.sciencemag.org/content/349/6255/1529

Uses alkaline pH and ferricyanide/ferrocyanide instead of acidic pH and Br2/Br-. You still wouldn't want to wash your hands in it, but it's significantly less hazardous than the previous chemistry. It's also compatible with more materials and less expensive materials. But the energy density, already modest, fell again with this easier-to-handle composition.

Later papers: a bunch of different variations on this theme. They're looking for higher energy density and better capacity retention, while avoiding corrosion/hazard issues. Higher energy density means smaller tanks and (usually) smaller membrane systems for a given energy storage or power output target. You don't want to lose all savings from inexpensive active molecules to a requirement for huge storage tanks and membranes.

This paper: claims a bunch of desirable properties. High efficiency. Better energy density than the original alkaline formulation. Long cycle life. Long calendar life -- meaning, the ability to sit unused for a long period and not lose too much capacity. (Several prior variants on this organic flow battery theme used persistent-radical chemistry for improved energy density, but even persistent radicals do not persist very well over a period of years.) This latest effort also retains the benefit that motivates this entire series: all required chemicals involve only very abundant elements.

For those unfamiliar with flow batteries, the most common chemistry is based on acidic solutions of vanadium. These flow batteries are fairly good on lifetime, efficiency, and energy density. But they do not store a lot of energy per unit mass of vanadium, and vanadium is fairly expensive. It hit a price of over $28/kg (as oxide) earlier this year. It does not seem practical to scale up vanadium flow battery manufacturing to tens or hundreds of GWh of storage capacity per year, because even if mass production made non-electrolyte components much cheaper, the vanadium alone would be too expensive.

That's why a lot of R&D is going into looking for alternative non-vanadium chemistries for flow batteries. Some of them seem like a lateral move from a bulk storage perspective. Cerium? Yeah, good luck scaling that to 100 GWh/year either. Others, based on abundant metals or on non-metallic organic molecules (like this effort), might have very good prospects for bulk stationary storage if all the kinks can be worked out.

This paper shows a full-cell flow battery concept with quite a few kinks straightened, which is why it's my pick for top battery paper in July.