Electrochemical storage devices were the first methods of harnessing electrical energy in the history of mankind. The remains of an Fe (iron) – Cu (copper) battery, dated back to 250 BC were found near Baghdad, Iraq in 1936. Archeologists believe that ancient civilisations, such as the Persian empire, may have mastered this type of water-based liquid battery and used it for electroplating thin metal coatings or for medical applications, such as the electric treatment of migraines and epilepsy. The remains found in Baghdad were from a primary battery (non-rechargeable) which operated via the galvanic corrosion (oxidation) of an iron rod (the anode) by the higher electrochemical potential of a rolled copper sheet cylinder (the cathode). The ionic connection between the anode and cathode was achieved by filling the can with acidic solutions such as vinegars, wines, or fruit juices. Electrical insulation between the two electrodes was achieved by an efficient asphalt plug, and the battery was enclosed in a clay jar the size of a beer can. It has been shown that such an electrochemical device could operate with a voltage between 0.5V and 0.7V, similar to the voltage of a modern fuel cell. Electrochemical generation of electricity was produced until the iron rod was “spent” (corroded completely) and could then be replaced to continue the process. The electrical current produced could be maximised by increasing the surface area of the two metal electrodes via abrasion and by using stronger acids as electrolytes. Vinegars or wines, which were widely available at the time, can provide high concentrations of acetic or tartaric acids. Unbelievably, ancient experts in this art may have had access to electrical power by connecting such batteries in series or parallel to realise larger battery packs with higher power outputs.
As new metals have been discovered, higher voltage batteries have been devised. While sources date the discovery of iron at around 5000 BC, it took humankind another 6,700 years to discover metallic zinc, which became available around the 1700s. As such, zinc (Zn) became the favorite anode metal for two hundred years due its easy corrosion in water acidic solutions and its low electrochemical potential (around -0.76V). Zinc offers a 0.3V advantage over iron, which has an electrochemical potential of -0.45V. Soon after its discovery, Volta showcased his 1V copper zinc “pile” battery in 1791. This famous primary battery used brine (solutions of table salt or sodium chloride in water) as the electrolyte and operated on the same galvanic principles as the Baghdad battery. In this case, zinc corroded (oxidised) as the anode under the influence of copper as the cathode.
As new metals have been discovered, higher voltage batteries have been devised
Primary batteries cannot be recharged without a manual replacement of the electrodes. However, secondary batteries benefit from a reversible electrochemical process where the anode is oxidised during discharge and can be reversibly reduced during charge, thus precluding the need for opening and replacing electrodes. Oxidation and reduction denote reversible electrochemical processes which correspond to a loss and a gain of electrons. Historically, oxidation has been associated with corrosion of metals. Rechargeable batteries were more rugged and opened the door for a wide area of long-term applications. The secondary lead acid battery and the Leclanché cell were both invented around 1850. The lead acid battery provides the highest voltage for a water-based battery (2V) and has survived with little fundamental changes to the present day, making it the most successful battery in recent history. It functions with a lead anode, a lead dioxide cathode, and a sulfuric acid electrolyte. The Leclanché battery operates at 1.4V and utilises a zinc anode and manganese dioxide cathode with an ammonium chloride water solution as electrolyte.
Around the turn of the 1900s, new batteries which used alkaline (or basic) water solutions (instead of acidic electrolytes) were invented. Nickel-cadmium utilised a potassium hydroxide solution and operated at 1.2V but provided a 50% improvement in energy stored per unit of mass (50 Wh/kg) on the popular lead acid battery (35 Wh/kg). The high price of cadmium incentivised Thomas Edison to develop the cheaper alkaline nickel-iron (iron as the anode) battery alternative which he patented in 1901 with the target of powering electric vehicles. Lewis Urry improved upon Edison’s alkaline battery to solve its major drawback – life span – by marrying Edison’s alkaline electrolyte with Leclanché electrodes. And so, the modern alkaline batteries, which pair an alkaline potassium hydroxide water electrolyte with a zinc anode and a manganese dioxide cathode, were born.
Lithium is nearly 15x lighter than iron, 13x lighter than zinc, 21x lighter than lead, and more than 4x lighter than graphite; It also provides the highest advantage in voltage
The highest energy-density water-containing rechargeable battery is called nickel-metal hydride (NiMH) and was first introduced in the satellite/aerospace markets in the 1970s. It has a nickel-oxide-hydroxide (NiOOH) cathode and hydrogen-absorbing alloy anode such as magnesium-nickel alloy. The electrolyte is a potassium hydroxide alkaline water solution. Even though this alkaline battery chemistry provides a lower voltage of 1.2V, it more than doubles the energy density (~100 Wh/kg) of the previous king – the lead acid battery. NiMH batteries represent the pinnacle of ruggedness, energy density, and safety of chemistries utilising liquid water electrolytes. While the energy density cannot go far above 100 Wh/kg, it is an intrinsically safe chemistry as the water-based electrolyte ensures these batteries will not burn if misused.
In an effort to significantly break through the 100 Wh/kg energy density frontier, Sony commercialised rechargeable lithium-ion batteries in 1991. These modern, high-energy-density batteries utilise a lithium-absorbing anode, such as graphite, paired with metal oxide-based cathodes such as lithium cobalt or nickel oxides (LCO, NCA, NCM, etc). Lithium ions migrate from the cathode and insert into the graphite anodes during charge, and re-insert at the cathode during discharge. To facilitate the higher charging voltage of 4.2V, water containing electrolytes cannot be used. Instead, solutions of lithium salts dissolved in flammable carbonate organic solvents are widely used. While providing more than 2x better energy density (275 Wh/kg in 2020) than the aqueous NiMH chemistry, Li-ion batteries with liquid organic electrolytes burn readily if malfunctioning and, by comparison, have serious safety drawbacks.
Over the past 10 years, as the energy density of Li-ion batteries has increased ~ 10%/year and the price has dropped more than 10x, society has adopted this transformational technology as an energy storage alternative in combination with solar panels and electric vehicles. While this trend has already begun, the industry desires a safe alternative to Li-ion without the low-energy-density drawbacks of water-based NiMH.
In 2020, we witness an important period in the history of batteries
Enter the solid-state battery, which enjoys the industry consensus of providing the path to the next 2x step jump in energy density (to above 600 Wh/kg) while providing an intrinsically safe chemistry, free of flammable liquid solvent. Solid-state batteries utilise solid lithium conductors (such as polymers, glasses, or ceramics) as electrolytes. They don’t burn and can be more chemically stable than water or organic solvents. In addition, they enable the use of more energetic electrodes and higher energy densities unachievable by traditional chemistries with organic solvent-based electrolytes. Solid lithium conductors available in 2020 are heavier than liquid electrolytes, so simply using the same electrodes common in Li-ion and replacing liquid electrolytes with a solid alternative creates batteries with lower energy density (albeit higher levels of safety) than commercial Li-ion. Based on the 2x energy density step jump principle, commercial adoption of a solid-state lithium-ion battery is thus governed by the successful marrying of more energetic lithium metal anodes and highly catalytic, high-voltage cathodes, such as nickel-rich or lithium-rich oxide/phosphate varieties. A lithium metal anode is the Holy Grail of battery anodes because it is the lightest anode material in the periodic table so it can store the most energy per unit of mass. Lithium is nearly 15x lighter than iron, 13x lighter than zinc, 21x lighter than lead, and more than 4x lighter than graphite. It also provides the highest advantage in voltage. The electrochemical potential of lithium is -3V which provides an advantage of 2.5V over iron and 2.25V over zinc. Overall, these two classes of electrodes are too reactive for the assembly of batteries which contain liquid electrolytes and have a high-cycle life. However, there is already evidence that varieties of polymers and light composite solid electrolytes may enable these challenging technical targets to be achieved.
In 2020, we witness an important period in the history of batteries – the advent of a new class of battery chemistry which offers yet another 2x step jump improvement in energy density. Such step jumps have occurred only four times in the past 250 years. The first such event occurred around 1850 when the lead acid battery (35 Wh/kg) more than doubled the energy density of jar-type batteries, such as the Daniell’s or the gravity cells. A second similar event occurred in 1901 when the nickel cadmium (Ni-Cd) battery provided a 50% improvement in energy density to 50 Wh/kg. Society had to wait until 1970 for the third event – the introduction of the nickel-hydrogen and nickel-metal hydride cells which opened the door to a 2x improvement in energy density to ~100 Wh/kg. The fourth event occurred in 1991 with the introduction of Li-ion which once again led to more than a 2x improvement (275 Wh/kg in 2020).
Solid state technology faces many technical and monetary challenges as well as opposition from the already functioning liquid electrolyte cell technology. We expect that this next generation battery will slowly solve these problems and start entering markets. The question is – how fast will this process occur?
