Our team has successfully synthesized a new stable allotrope of oxygen, tetraoxygen O4, with a boiling point of 43°C.
Patent   US-11548783-B2

O2 vs. O4

Low density in liquid state Twice the density of liquid oxygen
Average specific impulse with kerosene Highest specific impulse with kerosene, at least 20% higher than oxygen
Liquid oxygen boils and evaporates under normal conditions Tetraoxygen maintains liquid state up to 43 °C
Dangerously explosive in contact with rocket fuel even at low temperatures Safe to handle up to 200 °C, at which point decomposition into oxygen starts

Oxygen is used in many areas of human activity. Every year, millions of tons of oxygen are extracted from the atmospheric air and stored in a compressed and liquid state. Oxygen, however, has a low critical temperature (-118.38°C) and a high critical pressure due to its low molecular mass, making its long-term storage quite challenging and expensive. There are several known allotropic modifications of oxygen: triplet oxygen (hereinafter simply "oxygen"), singlet oxygen and ozone. Triplet oxygen O2 is a gas that boils at -183°C and solidifies at -218°C. It is known that another allotropic modifications of oxygen, singlet oxygen, has very different physicochemical properties than atmospheric oxygen. Singlet oxygen is produced chemically by the oxidation of peroxides with chlorine. It exists in this state for several tenths of a second, and then decomposes to ordinary (triplet) oxygen with the release of infrared radiation.

Another allotropic modification of oxygen - ozone, molecules of which comprises three oxygen atoms, sometimes forms naturally.

Chemical and physical properties of ozone O3 differ significantly from those of molecular (triplet) oxygen O2. Boiling point of liquid ozone is –112°C. Boiling point of liquid oxygen is −183°C. Density of gaseous and liquid ozone is 1.5 times that of atmospheric oxygen. Ozone decomposes into molecular O2 in a few tens of minutes. In concentrated form (more than 70% by volume), ozone can explode spontaneously, releasing a large amount of energy. Therefore, long-term storage of ozone is impossible.

Our team has invented a method, designed and built a device for production of a new long-term storage-stable allotropic modification of oxygen, tetraoxygen O4, using a combination of known chemical reactions into one technological sequence, including chemical interaction of negative and positive oxidation state oxygen compounds.

The method involves production of dioxygen difluoride by oxidation of molecular oxygen with fluorine, followed by the reaction of dioxygen difluoride with alkali metal peroxide, forming tetraoxygen O4.

Tetraoxygen is stable in its liquid state up to a temperature of around +43°C and can be used for the oxidation of rocket fuel, long-term compact storage of oxygen, and many other purposes. 

Tetraoxygen is not toxic.

It is an ideal rocket fuel oxidizer because it is not toxic, has low vapor pressure, high density in liquid state. When the fuel is oxidized, tetraoxygen forms the same substances as molecular oxygen. The stability of the tetraoxygen molecule is explained by the fact that all oxygen atoms in it are surrounded by octet of electrons, and each atom is associated with two neighboring atoms by common electron pairs of paired electrons. Moreover, all atoms in tetraoxygen are equivalent and closed in a ring.